[aspundir]

The sudoku board (as seen above) is made up of 9 squares creating a 3 x 3 grid. You will also notice that each of these 9 squares are further divided into 9 smaller squares which are called cells, creating smaller 3 x 3 grids. Each and every cell holds a value from 1 - 9, and it is up to you to determine what values they are - this is the whole concept of the game.
There are restrictions however to what number can be placed in what cell as every square (that is also a 3 x 3 grid) must contain all numbers 1 - 9, and as there are only 9 cells in each square - you've most likely realised that you can not have two of the same number if you are to include all the numbers 1 - 9. For example: a grid containing numbers 1, 2, 3, 4, 4, 5, 6, 7, 8 would not be allowed. There are 9 numbers granted, however there contain two number fours and so would be incorrect.


Also that is not the only restriction to where a number can be placed. You may or may not have noticed but each column and row contain 9 cells (not by coincidence), and each of them must also contain the numbers 1 - 9. This means that each column and row (if done correctly) will never have the same number in twice.
You will need to look at every cell and see if a number can go there, you will be able to see if a number goes there as other numbers (in columns/rows/square) will rule out possibilities.


Have a look at the top-left square from the images that are present. You will see that it contains numbers 3, 5, 6, 8, and 9. This means that these numbers will not be able to be included in this large square as you must have all of the numbers 1 - 9. This means the numbers 1, 2, 4, and 7 are the only possibilities for the remaining four cells in that square. You must look at each of those cells and determine if numbers in other cells (from rows/columns) rule out possible cells for that number to go until you are left with only one possible cell for it to go.