Can I say “He cries when slapped by his friend” [closed]

If it is correct, could you tell me what grammar topic should I review to develop my knowledge?

It's fine grammatically; you can interpret it in either of two ways: (a) a narrative - say, as part of a story ("When Charlie isn't looking he takes the fruit. He cries when slapped by his friend. He returns the fruit."); (b) as a type of (zero) condtional, equivalent to 'He cries whenever he is slapped by his friend'.

Suggested grammar topics: conditionals (especially the zero conditional); ellipsis (that is, the rules for omitting "he is" in the original sentence); the passive (his friend slaps him -> he is slapped by his friend)