When is used in asking questions.

–When are you leaving?

When can also be used in a dependent clause:

–I forgot the date when you arrived.

In the latter example, when is used to refer to a noun of time (i.e., a day, week, month, etc.)

In time clauses, it is also possible to use that or which preceded by a preposition:

–I forgot the date that you arrived.

–I forgot the date on which you arrived.

No preposition is needed with that.

Note how two sentences are combined using when:

–I'll always remember the day she was born. She was born then (on that day).

–I'll always remember the day when she was born.

Examples of Using When

• When it comes by, wave and the driver will pick you up.