When (adverb)

1. At what time.
2. During the time that.
3. At the same time that.
4. At any time.
5. At some indefinite time in the future.

Origin:

Middle english, from old english hwǣnne; akin to old high german hwanne when, latin quando when.

Examples:

1. When will you be back?
2. When did you see him?
3. I'll call you when I get there.
4. When will it stop raining?
5. I'll tell you when I'm ready.