The Basics:
Before diving into strategy here are basic concepts and rules of a sudoku board: Sudoku grid consists of 81 square tile divided into none columns named with letters a to i and 9 rows from 1 to 9 (kind of like chess but one row/column less) It is also divided into nine 3 by 3 sub grid block. Every row, column, block contains numbers from 1 to 9 without repetition (ony diagonals do not count in classic sudoku.)
Sudoku is a great example of the magic of numbers - the whole board can have only one solution, and for every single tile there is only one possible number to place.
Board scanning
Easy: scanning. in most easy/medium boards you can scan rows, columns and blocks to find the spots where you can already place only one figure:
1. Scan for number repetitions on the table.
You need to find the numbers with the biggest count currently on the table (Most advanced sudoku games can do that for you - just tap the right number - and all occurrences will be highlighted on the table). See image below: we have 1 repeated 4 times, 2: 5 times, and 3: 5 times.
2. Scan in one direction.
You can already place missing number in the bottom right block by simply scanning 3 bottom rows: row 7 and 9 already have 3 and the final row 9 has only 1 empty cell in bottom right corner.
3. Scan in 2 dimensions: both vertically and horizontally
See another example, columns A and B have 3 placed already, and row C has 2 possible placements. In this case you need to scan horizontally. Apparently row 7 already has 3, giving you only one possible opportunities.
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Scan for single candidates.
As long as number cannot be repeated neither horizontally, nor vertically, nor i a block. Whenever you spot a number surrounded by another 8, you already got the solution.
See the example above and check what options have you got:
2 - in the column & the block
3 - in the column
4 - possible
5 - in the column
6 - in the block
7 - in the row
8 - in the row
9 - in the column
At the end you can fill this sudoku cell with only number 4. Profit!
- Single elimination Whenever a row, column or block is missing 2-3 numbers, you can scan for elimination opportunities. At the example below only 5 and 9 are missing in the column F. However F1 cannot contain as it already exists in the block - therefore only 5 can be placed there, which leaves us with 9 at F6