Time clauses in English Grammar

Adverbial clauses of time consist of two clauses: the main clause and the time clause. The time clauses begin with: when, whenever, while, as, since, after, before, until, as soon as, once. The time clause may come before or after the main clause with no change in meaning. If they refer to the future, the Simple Present is used in the time clause, and shall/will + infinitive/perfect infinitive or modal + infinitive, or imperative is used in the main clause.

When a complex clause contains a temporal clause, the time clause has future reference. The future is normally indicated by will/shall ('shall' with 'I/we') but not in the subordinate clause.

Peter will return soon. Then he will telephone you.
Peter will telephone you when / as soon as he returns.
That has a future reference. Future is marked by the main clause.

We may use Present Simple or Present Perfect when we expect future.

I will return your book on Monday. I will have read it by then.
We can combine these two sentences into a complex sentence in the following way:
I will return your book on Monday when / as soon as I have read it.

The rule stated for a complex sentence containing a time clause will apply to temporal clause with future reference introduced by: after, before, till, until, whenever and by other expressions which could replace when and as soon as like e.g. once, immediately, the moment, the minute, the day
He will call you immediately / the moment he gets home.
Once you've seen one of his pictures, you've seen them all.

The main clause could be imperative.
Come and see me me as soon as you have a spare minute.

Till / Until

The two words are inter-replacable and they mark the end point of the period of time and they are associated with the verb denoting an action or lack of action which can continue during the period ending at that point.

Wait till I come / I return.
Don't leave / You mustn't leave Waiting will continue throughout the period. 'Leave' and 'go' in the affirmative could not replace 'wait' (in the first sentence) since they denote actions performed at the point of time not continuing through the period.

Since

When it introduces a time clause, it can mark the beginning of the period of time which continues until now or until then. In temporal clause a verb in the past can mark the beginning of the period while the verb in the main clause is Present Perfect of the period which continues until now or Past Perfect if it continued till then (in the past).

Since I left school (till now), I've only seen him once.
I met Peter last week since we left school (till then), we had often written to each other.

And 'since' followed by a verb in Present or Present Perfect referring to activity having during and still continuing means: 'during the period when'.

Since we've left we met many people.
We cannot replace 'left' by 'came' because it refers to an activity.

While

It can mean 'during the period that'
A. The postman arrived while I was having a bath.
B. I was doing my homeowork while the children were playing.
C. Peter waited while I had a bath.

In A and B 'while' can be replaced by 'when' and 'as' but 'while' C could be replaced by 'as' but not usually by 'when' ( 'when' refers to definite time that ... )

I cut myself when I was shaving.
The robber was arrested as e was leaving the bank.