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When I ... (leave) London next month I will have studied here almost a year Why is the answer leave and not am leaving

"Am leaving" in this case expresses the future, so "am leaving" should be good,all the more that leaving can be a process.

I don't understand why "am leaving" is not good

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    We can say "I am leaving London next month" to express the future. However, your sentence refers to the departure as a specific point in time, and we don't use the continuous tense in that context. It may seem illogical to you, but it's how the language works. – Kate Bunting Dec 24 '20 at 9:19
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    is it because of when that you consider the leaving as a specific point of time – Yves Lefol Dec 24 '20 at 11:58
  • Yves, if you don't use a question mark it isn't clear that you are asking a question! The same idea can be expressed without using 'when'. The day the speaker leaves London will be almost a year after their arrival there. – Kate Bunting Dec 24 '20 at 15:31
  • I can't explain why am leaving doesn't work in this context (it's so bad I'd guess it's syntactically invalid, rather than just non-idiomatic). But it's worth noting that in this very similar context you can use either Simple Present OR Present Continuous (with auxiliary TO BE): I feel more positive when the sun *shines / is shining. Why the same doesn't apply to the example here is beyond me. – FumbleFingers Dec 24 '20 at 16:18
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This is honestly a tricky one to figure out. Here's the best explanation I can offer:

The use of "when" is the reason why "am leaving" is unnatural. We could structure the sentence as two independent clauses, and it would be perfectly acceptable:

I am leaving London next month, and I will have studied here for almost a year.

But "am leaving" becomes unnatural when we use it inside a subordinate clause with "when". The simplest way I can make sense of this is that there are a couple different types of subordinating conjunctions, and they like to use different tenses to express future events:

  • The first group is time-related conjunctions, like when, once, before, after. Subordinate clauses using these have a bias for the simple present tense. It's natural to say, "When I leave London... once I arrive in New York... before I go to the hotel... after I eat dinner". The pattern here is that these refer to what's happening at a specific moment in time. (The action in every case does take time to complete, but that's not grammatically meaningful here.)
  • The second group consists of what I'm going to call "logic-related conjunctions" (maybe there's a more formal term... I don't know). These include because, hence, therefore, and so on. These have a bias for the present continuous tense: "Because I am leaving London next month, I can't come to your wedding in July", or "I got a job in New York, therefore I am leaving London".
  • There are also some conjunctions that can go either way... sometimes adding different shades of meaning: as, unless, while, since. Interestingly, most of these words can have time meanings or logical meanings. So, we might say, "I'll wave goodbye as I board the plane," (time meaning = simple present), and also say, "I can't come to your wedding, as I'm moving to New York." (logical meaning = present continuous).

The big exception to the first group (time-related conjunctions) is when you're deliberately referring to something happening during an event. Here the present continuous is appropriate. For example:

When I am writing my thesis next month, I won't have time for Netflix.

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