Is present continuous with when not possible here

Question

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

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 09:19
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
See Construction of future tense with 'when' and “when you do something” or “when you are doing something”? — FumbleFingers, Dec 24 '20 at 16:24

score 0 · Accepted Answer · answered Jan 08 '21 at 22:25

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.

Is present continuous with when not possible here

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