[aspundir]

You won't be able to determine where numbers can go straight-away and will require you to travel around the sudoku board searching.
Let's number every cell in each square A - I just so I can show you some examples. We're number them A - I as you would read a book. So looking at the first square; cell A contains the number 5, cell B contains the number 3, cell C is empty, cell D contains the number 6, cell E is empty, cell F is also empty, cell G is also empty, cell H contains the number 9, and cell I contains the number 8.


Let's try looking for the number 1 in the top-left square. The only other number 1 that would have an impact to our square is the top-middle square that contains the number 1 in cell D. This rules out the possibility of the number 1 being in cells E and F in our top-left square, and leave us with two possibilities to where the number 1 could be: cells C or G. We don't have enough numbers to determine which one so we'll have to come back to that later.
Some people like to pencil in small the possible numbers which could occupy that cell.


Now let's try looking for the number 2 in the top-left square. You will notice that there is not a single number 2 that is in a line or column that affects our top-left square, so at the moment the number 2 could potentially occupy any of the four cells (C, E, F, or G) in our top-left square.


Now let's try looking for the number 4 in the top-left square. You will notice that there is a 4 in the middle-left square (cell D) that does affect us, and it rules out cell G in the top-left square to where the number 4 could appear. However it is the only cell that does affect us and so the number 4 could potentially still be in any of the cells C, E, or F.


I'll quickly do the number 7 for the top-left square too. The number 7 appears in the top-middle square and rules out cell C in the top-left square to where 7 can appear - this leaves us with cells E, F or G.
There is also a number 7 however in the middle-left square and that rules out cell G in the top-left square. Now only cells E and F remain to where the number 7 can appear. We don't have any more information at the moment so we'll have to move onto another square.


I'll now show you an example where a number can be found. Look at the bottom-left square. It only contains the number 6 and so can still house the numbers 1, 2, 3, 4, 5, 7, 8, and 9.
Let's try looking for the number 8. The number 8 can appear in any of the cells A, C, D, E, F, G, H, or I. You will see however that there is a number 8 in the top-left square and that rules out cells C, F, and I straight away, leaving cells A, D, E, G, and H.
Then you will see another number 8 in the middle-left square and that rules out cells A, D, and G in our bottom-left square - and that now leaves us with cells E and H, both of which could potentially hold the number 8.
There is also another 8 in the bottom-middle square and that rules out cell H for us, which only leaves cell E. As there is only one possibility for where the number 8 can go (cell E) that means that the number 8 HAS to go there. It doesn't make any difference that there is a number 8 in the bottom-right square as that rules out cells A and C that the previous number eights did (in top-left and middle-left squares.