Use when meaning at the time that to join a time clause to the action it anchors: the when-part is a subordinate time clause, so it needs a complete main sentence beside it. We left when the rain stopped. Here when the rain stopped is the time clause and We left is the main clause.
- If your draft stops at When the rain stopped, it's incomplete; add a main clause like We left.
Asking about a time with 'when'
*When* works as an adverb when you want to know the time of an action. How you word the question depends on whether you ask someone directly or pass the question on:
You're joining two events in time. When meaning 'at the time that' links a subordinate clause to a main clause, and the subordinate part can't stand alone: When the rain stopped, we went out. The clause when the rain stopped answers at what time went out happened, so it works as an adverbial modifier of the main verb but depends on the main clause for its full meaning.
You're joining two events in one sentence. Here when works as a conjunction meaning 'at the time that', and the condition is that the when-clause must attach to a main clause; on its own it can't stand as a complete sentence. So you say I'll call you when I land, where when I land completes the main clause. Note that for future events the when-clause takes present simple or present perfect, not will: I'll call you when I land, not I'll call you when I will land.
You're asking about a starting point. When still acts as an adverb, often paired with since: Since when do you check my drafts? The time question reaches into the present.
You're attaching the when-clause to a main clause in a statement. The when-clause can't stand alone; it needs a main clause to complete its meaning: She asked when the train arrived. Here when the train arrived completes asked, so drop the inversion and the auxiliary stays after the subject, with no did. In a standalone direct question like When did the train arrive? the wording changes, but once the question is folded into a statement, the when-clause follows normal statement word order.
You're attaching a when-clause to a main clause that points to the future. Match the tense to the original timing: the when-clause uses present simple or present perfect, and the future stays in the main clause. I'll email you when the report is ready, not when the report will be ready. This tense choice matters in statements rather than direct questions, so the adverb moves into the clause without did or any auxiliary inversion.
The boundary worth checking before you write: when you attach a when-clause to a main clause, the when-clause keeps normal word order. Front position plus auxiliary inversion belongs only to a direct question, so it never appears inside the joined clause. That's why Tell me when the meeting ends stays flat even though it asks about a time: the question is embedded in a statement, so it takes statement order. If you find yourself writing inversion inside the when-clause, that's the signal you've shifted into direct-question wording, and a joined clause won't carry it.
When you want the time part first, you can put the when-clause at the front: We left when the rain stopped is one order, and When the rain stopped, we left is another. Either way, this is when joining two clauses, one naming a time and the other telling what happened at that time. The time clause can go first or second; the meaning stays the same.
Condition: the when-clause starts the sentence, as in When the rain stopped, we left. Put a comma after the time clause to separate it from the main clause.
When the when-clause comes after the main clause, as in We left when the rain stopped, no comma is needed.
One boundary: the comma follows position, not length. Even a short front clause takes the comma, and a long ending clause still goes without one.