Your first Sudoku move
Start with what you already know.
An almost full row
Count the given numbers in a row. If only one is missing, you have a candidate. Check whether it fits the empty square's column and 3 × 3 box.
One square, one possibility
If several squares are empty, examine one. Rule out numbers already in its row, column or box. If one number remains, that is a safe move.
When you get stuck
Use Explain a move. For an incorrect entry, it can show a correction from the solution. Otherwise, it explains a step using the visible numbers. You choose whether to enter it.