The noun sentence (a text form, or a prison term) requires an adjective or article (a, the). But you can use it as an adjective itself, or as an imperative of the verb to sentence, to announce a...
For an answer to a question. "What time do we have to go to bed?" "WHEN the clock has the numbers 9,0,0." When as a question word: When did you live in Australia? When as a conjunction: We were...