when's

The everyday reading of the contraction, and the only one that carries no informal label. In when's the next train?, a noun phrase follows directly, and that is the sign of is. The same 's can also stand for has, does or was, so the next word is the clue. A participle points to the "has" reading, and a bare verb points to the "does" reading.

Here the 's is the auxiliary has, so a past participle comes next: when's he said he's coming? It is most at home in rhetorical questions in the perfect, such as when's he ever finished a project on time?, where ever carries the reproach. It sits comfortably in informal speech. In careful writing, spell out when has, since a reader may first hear is.

Does shrinks to a bare s mainly in rapid speech, and the verb after it keeps its plain form: when's the next train arrive? Set that beside the "is" reading, when's the next train?, and you can see how one added verb changes the hidden auxiliary. It belongs to informal talk. In prose, write when does.

A past-tense reading that is colloquial and mostly spoken. It typically turns up in when's the last time…? Nothing in the 's itself signals was rather than is, so the surrounding words have to carry the tense, and a question about the past can sound like one about the present. In writing, when was removes the doubt.