Say what you would do if…(Conditional)
If I (to see) some schoolboys beating a smaller boy …
If a child (to ask) me to help …
If I (to hear) a person tell a joke about my best friend …

If I saw some schoolboys beating a smaller boy I would tell them to stop.If a child asked me to help I would ask what's wrong.If I heard a person tell a joke about my best friend I would defend my best friend.