1. Questions about time:
* When will you be home?
* When did you last see her?
2. Clauses that describe a time or condition:
* I will go to the store when I finish my work.
* We'll leave when the rain stops.
3. Clauses that express a reason:
* I'm happy when I see my friends.
* She gets upset when people interrupt her.
4. Relative clauses:
* This is the time when everything changed.
* The place when we met is still there.
Examples:
* When is the party?
* I'll call you when I get there.
* I was surprised when I saw her.
* This is the book when I learned about history.
Note: "When" can also be used as an adverb, meaning "at what time" or "at the time that."
* I saw her when I was in the store.