8.20 saying when a situation ends

If you want to say that a situation stopped when something happened, you use 'until' or 'till'.

You also use 'until' or 'till' to say that a situation will stop when something happens in the future. In the time clause you use the simple present tense or the present perfect tense.

8.21

'By which time', 'at which point', 'after which', 'whereupon', and 'upon which' are also used at the beginning of time clauses.

You use 'by which time' to say that something had already happened or will already have happened before the event you have just mentioned.

You use 'at which point' to say that something happened immediately after the event you have just mentioned.

You use 'after which' to say that a situation started to exist or will start to exist after the event you have just mentioned.

You use 'whereupon' or 'upon which' to say that something happened immediately after the event you have just mentioned and was a result of it.

8.22

You can use a clause beginning with 'when' after a question beginning with 'why'. For example, you can say 'Why should I help her when she never helps me?' However this clause is not a time clause. In your question, you are expressing surprise or disagreement at something that has been said, and the 'when'-clause indicates the reason for your surprise or disagreement.