As a conjunction to introduce a subordinate clause that shows time:
* When I get home, I'm going to take a nap.
* We went to the beach when the weather was warm.
As an interrogative adverb to ask about time:
* When will you be back?
* When did you last see her?
As a conjunction introducing a dependent clause that describes a situation or condition:
* You can go to the party when you finish your homework.
* I'll help you when you need it.
In combination with "if" to introduce a conditional clause:
* When if ever will you learn to be responsible?
To emphasize a particular time or event:
* I saw him when I was at the store.
* When, exactly, did you decide to move?
In informal speech, "when" can be used to mean "if" or "whenever":
* When you need anything, just ask. (meaning "If you need anything...")
* I'll call you when I get there. (meaning "Whenever I get there...")
The meaning of "when" will depend on the context of the sentence.