Sudoku Is Hard Until You Understand These 3 Concepts

Added: 2026-05-25

[00:00:01]Have you ever considered solving a puzzle backwards?

[00:00:04]I will show you three Sudoku concepts that solve puzzles like this by defying conventional logic. And with that, it's solving time. So, concept one is to consider advanced strategies at the beginning. If you look at these ones here and mark where all the possibilities are for one, not just in each 3x3 block, you'll notice you have this pattern of ones in the puzzle.

[00:00:37]Now, if you color those in green, what you might notice in row one, row six, and row nine, the ones are restricted to the same three columns, columns two, four, and eight. This is called Sudoku swordfish. And since the ones are restricted to the same three columns in these three rows, you cannot have additional ones in those columns. If you put ones in any of these spots, you'd end up breaking the puzzle. So, you can remove the ones from right there, and now you're able to mark where all the ones can be by using an advanced strategy at the very beginning of this puzzle. It's going to get you so much further. Greetings, friend.

[00:01:27]This puzzle by Jovial is perfect to show you three powerful concepts that make it okay to sometimes break the normal solving rules. So, concept one is to consider advanced strategies at the beginning. Concept two is so easily overlooked that I've dedicated dozens of videos to show the importance of it. Concept two is to look for intermediate solves. And what I mean by that is you don't want to go from looking for advanced strategy to another advanced strategy. So many times you have to set them up with these critical easy to medium strategies.

[00:02:05]So, if you notice in row four, you have a two and three here, and you have a two and a three there.

[00:02:11]By using double scanning, where can the two-three be here in block five?

[00:02:15]They're restricted to these two cells.

[00:02:17]And so, this is a Sudoku hidden pair of a two and a three.

[00:02:23]Since the two and three have to be in these two cells, nothing else can be in those cells. You got to find that. You can do the same thing with the seven and the eight. The seven-eight in the row in the columns that I just showed you restricts the sevens and the eights to these two cells right here, which just leaves you with the nine and the six in block five. And you have to find this to set up the next advanced strategy cuz you got to remove some of the sixes that were possible in block five. And now, the question of the day, what is your solving style? Do you like to go from easiest to hardest strategies, lots of marks or no marking at all, are you slow and careful, or do you race to solve as fast as you can? Please Please share in the comments. Help me grow the internet's best Sudoku community.

[00:03:12]Okay, so bring you up to concept number three. And this is something that's easily messed up or mistaken. Most people do not understand the idea. I'm going to show it to you. And it's the concept of strong and weak links.

[00:03:26]If you were to look at this puzzle and try to figure out setter's intent, what did Jovial want you to find? You might notice that you just restricted the sixes in this block. And you have these sixes on the corner. So, it restricts the sixes to these cells here in blocks one and block nine. It make you want to look and see where are all the sixes possible in this puzzle.

[00:03:52]Okay?

[00:03:53]So, you can mark all these up.

[00:03:58]And I'll put them in green. And then, the backbone for advanced strategies, even things like swordfish that you I just showed you, is conjugate pairs. And before I get too far, I do want to make sure I cover that block as well.

[00:04:13]So, a conjugate pair is simply when you have two possibilities for a candidate in a row, column, or block. So, this is a conjugate pair. If this is a either a six, if it's not a six, this cell has to be a six.

[00:04:25]Okay? This is a conjugate pair in the row of sixes. It's either here or here in the block here or here, and in this block right there. A lot of conjugate pairs.

[00:04:35]And they're also called a strong link.

[00:04:38]And so, it's the same thing. A conjugate pair has a strong link. And what it means is if one of these cells is false, the other has to be true. That's all a conjugate pair strong link. That's all it means.

[00:04:51]Now, when you connect these to weak links, you can create very powerful solving strategies called alternate inference.

[00:04:59]Okay? And you're going to alternate from strong to weak link. So, a weak link means if a certain candidate is true for a cell, then everything else that it sees has to be false. So, if this cell right here was a six, then this cannot be a six, those cannot be sixes, those cannot be sixes.

[00:05:21]And so, you alternate between strong and weak link. That's as easy as it gets.

[00:05:25]Now, you might be wondering, "Wait, Timberlake, you just said this was a strong link, but now you're saying it also has a weak link." That is true.

[00:05:31]Here's the thing about strong links.

[00:05:34]They also act as weak links. Cuz the same thing's true. If this is false, that's true. So, it makes it a strong link. But if this is true, then this cell would be false. It could not be a six. So, you can substitute these strong links throughout your chains. But the main thing here is weak links allow you to have rows, columns, and blocks that have more than two possibilities.

[00:05:56]Now, combine these together. I want you to start right here.

[00:05:59]And I'll put in red everything that's part of the chain.

[00:06:03]So, you start with a strong link, and you have to end in a strong link. And you alternate between strong and weak as you go. So, if you notice right here, this has a strong link with this cell.

[00:06:12]Right? And then, you want to find a weak linked cell that will connect it to another strong link. So, none of these do that, but this cell does. Right?

[00:06:21]Because it has a strong link there. So, that's where you want to go next. So, this is weak, and this would be your strong link.

[00:06:29]All right. And if you're from here, where do you want to go? Well, you want to find another weak linked cell that's going to get you to a strong. And you notice in row five, there's a strong link. So, you want to go down here.

[00:06:38]Weak link there. To a strong link right here.

[00:06:42]And then, from here, where do you want to go? Again, try to find a weak link that's going to get you to a strong link. You have a strong link here. So, you want to go to this cell next and end up right there.

[00:06:56]So, remember, we started in this cell.

[00:06:58]So, you have a strong link here, weak to this cell, strong to this cell, weak to this cell, strong to this cell, weak to this cell, and strong to this cell right here.

[00:07:10]What that tells you is this is either a six, if it's not a six, you put a six there, not there, a six there, not there, a six there, not there, a six right there. With the ending and beginning in strong links, this cell has to be a six, or this cell has to be a six. No other possibility.

[00:07:29]It's like an extended conjugate pair.

[00:07:32]And what that tells you then, you can eliminate a six from any cell that sees both. So, shares the row block here, or shares the row block here. So, you can eliminate all those colors. And what that does is now restricts the six to one possibility in block nine. And I'm going to tell you, you can solve this cell now for a six, but you're not done with the advanced strategies. In fact, you will need to use this idea strong and weak links again. But first, I want you to know that you may be able to solve hard puzzles like this easier than you think. I cover the easiest to the most advanced Sudoku strategies in my 30-day Sudoku challenge. And the real value is you can dementia proof your brain for up to 20 years. Course participants love that they have lifetime access to each lesson and are able to learn at their own pace. So, click on the pinned comment to start dementia proofing your brain today.

[00:08:34]So, you solve the six right here.

[00:08:37]Not done with the sixes yet.

[00:08:39]You can mark sixes right here.

[00:08:42]Let's get rid of all the reds cuz you're going to continue to use these green cells. You can mark sixes right here. They're restricted to two spots. You can mark sixes right there.

[00:08:56]They're in two spots.

[00:08:58]But you notice now you created another conjugate pair right here. Right?

[00:09:02]Cuz sixes can't be here anymore. Cannot be here anymore.

[00:09:07]So, if you look right here and use strong weak links, you can go either way, actually. You can go over here and say, "Hey, that's a strong link." cuz there's only two possibilities in row four. Weak link up here cuz you got three possibilities. And then, another strong link right there.

[00:09:23]So, this has a strong weak strong. Four cells. This is the smallest form of an X-chain that I just showed you.

[00:09:32]And it looks kind of like a funky two-string kite or skyscraper. This is a turbot fish. So, all four-cell alternate chains to all two-string kites and all skyscrapers are type of turbot fish. But if it doesn't fit one of those neat patterns, it's just a turbot fish, like this one. So, you know this is a six, if it's not a six, that'd be a six, that can't be, this would have to be a six. You have to have a six in one of these cells. So, you can eliminate a six from right here.

[00:10:00]Beautiful, beautiful stuff that Jovial put in this puzzle to let you find out.

[00:10:05]And so, what does that mean now for the sixes? Where can they be?

[00:10:08]In column four. Only in one spot now. You have to solve this cell for a six cuz a six can't be anywhere else because of the you now learn how strong and weak links learn X chains and turbot fish work and skyscrapers. So, what kind of reward is Jovial give you for finding these two sixes? That's what you have to figure out.

[00:10:30]And the reward is actually in this cell right here.

[00:10:34]What can this cell be?

[00:10:36]Well, first you notice with the six there that's got to be a nine. This cannot be a two, three, four, five, six, a seven, an eight, or a nine now because of the solves you made. This has to be a one. And now you remember that swordfish at the beginning of the puzzle?

[00:10:53]Because you made those marks and then out this puzzle, you know, that has to be a one displacing that one, that has to be a one displacing this one, that has to be a one displacing that six also displacing this one, that has to be a one displacing the one there, and you can solve all of the ones. Beautiful.

[00:11:11]But you're not done. There's so many empty spaces. So, where do you go now?

[00:11:17]Well, start here in column six. You have now have a one, two, three, four, nine, and a five, six, seven, and eight. Well, with the five, six, eight right here, this has to be your seven, which means this is going to be an eight. And it leaves just a five, six naked pair in column six. Okay, with this eight, these two eights means this is an eight, and now you see that has to be a seven.

[00:11:41]Just gives you a nice five, nine naked pair there to finish row four.

[00:11:46]All right. And now with these nines, you can solve for a nine right here. If you look at this cell, what can it be?

[00:11:54]Can't be a one or two. Cannot be a four, five, six, seven, eight, or nine. This is a naked single three. So, you can solve that for a naked single three.

[00:12:05]Leaves you with just a nice four, six naked pair right here. So, we're kind of going backwards. After the advanced strategies, now I'm adding the pairs and triples. Isn't that crazy?

[00:12:14]With this three now, that's a two, that's going to be a three. And since one of these has to be a six, that can't be a six anymore. So, that's got to be a six right there. With this seven, that's got to be a seven, leaves just a two here in block six.

[00:12:28]Okay, after you made those solves, follow these twos. With these twos, that has to be a two right there. And then with that two, you can solve for a two here in block two.

[00:12:36]Follow these twos and this two to solve for a two here in block one. And then with follow those twos and this two, solve your last two here in block seven.

[00:12:47]And now look at a nice heavy house here in column five. You have a one, two, three, five, six, seven, eight. You need just a four and an eight. Well, with the eight right here, you know, that's got to be a four.

[00:12:59]That's got to be your eight. Leaves just a four here in block two.

[00:13:03]And then you just have looks like a three and a nine to finish up this column. And where do you go from there?

[00:13:10]And then look over here in column two.

[00:13:12]You might notice with these two nines, you can solve for a nine right there.

[00:13:16]And since the three can't be here and it can't be here cuz of this three, that leaves you a three right there displacing that six.

[00:13:24]As you solve now this for the four, that's your six, that's going to be your four now. And then you just have a five and an eight here.

[00:13:32]Pull that eight over.

[00:13:34]There's your eight, that's going to be your five. Disambiguate the six and five here. Getting so many solves, so keep it up. You might notice here in row three, you just need a four and a nine. We'll pull that four up. So, there's your four and there's your nine. Now, gobble up those marks.

[00:13:49]With that nine, that's a five, and that's going to be a nine right there. What do you have left here? Looks like you just need a four and an eight. Well, you have this eight here.

[00:14:00]So, there's your eight, and that's going to be your four. And then you can see with these fours, you can solve for the four right here. Leaves you with just a three and a five. Can't solve that yet, so just make you know, some quick marks there to help you out.

[00:14:17]Looks like you just need a five and seven here. Well, this five's going to help you out. So, there's your five, and that's going to be your seven. And you just need a looks like a three and an eight there.

[00:14:27]With this three, there's your three, and there's your eight right there.

[00:14:31]Always solve a full house. Just missing an eight, so you can solve that for an eight now.

[00:14:36]And then you'll notice here, you just need looks like a seven and a nine. Pull that nine over. So, there's your nine, there's your seven. Follow these sevens over, solve that for a seven.

[00:14:47]This nine, that's got to be your three, that's got to be your nine.

[00:14:50]And now with this three, you can disambiguate the five and the three right there. Go over here to block seven, that's got to be your three. And your last cell is a five.

[00:14:59]Now, challenge yourself to use a concept you just learned to solve these next puzzles. Thank you so much for watching.