Why can't I say I will call you when I will arrive?

Because the word when already tells the listener the action is in the future, so English refuses to mark it twice. After a time word (when, while, as soon as, until, before, after, once, by the time) you use a present tense even though you mean the future: I will call you when I arrive. The will stays in the main clause. Almost every language that has this rule breaks it differently, which is why the mistake survives so long.

What is the difference between when and while?

When points at a moment or a whole event: when the bell rang, when I was young. While points at a stretch of time that something else happens inside: while I was cooking, the phone rang. A useful test is the verb form. While usually takes a continuous verb (while she was reading), and when usually takes a simple one (when she read the letter). If both actions are long and happen together, while is the better word.

What is the difference between until and by?

Until marks a stretch of time that runs up to a point and then stops: I waited until six means the waiting filled every minute. By marks a deadline: finish it by six means at six or any time earlier. Until goes with things that continue, by goes with things that get completed, which is why the future perfect pairs with by (by six I will have finished) and almost never with until.

Is it ever correct to use will after when?

Yes, when when is a question word rather than a time conjunction. In an indirect question it introduces a noun clause, not a time clause, and the normal future rules apply: I don't know when he will arrive. Nobody can tell us when the shop will open. Compare I will phone you when he arrives, where when joins two events in time and takes the present.

Time clauses in English: when, while, as soon as, until, before and after

A time clause tells you when the main action happens. It is half a sentence, not a whole one, and it starts with a time word: when, while, as soon as, until, before, after, once, by the time. On its own it dangles, waiting for the rest: when the bell rings, and then what? Join it to a main clause and it works: when the bell rings, everybody stands up.

The grammar itself is not hard. One rule inside it causes more errors than any other single point at B1 level, and we will get to it in the next section because it is the reason most people arrive on this page.

The rule everyone breaks: no will in the time clause

English marks the future once. If a time word has already told the listener that we are looking forward, the verb after it stays in a present tense.

I'll call you when I arrive. (not when I will arrive)
We'll wait until the rain stops. (not until the rain will stop)
As soon as she gets here, we'll eat. (not as soon as she will get here)
By the time you read this, I'll be gone. (not by the time you will read this)

Look at where the will went in each pair: into the main clause, where it belongs. The two halves are not both marked for the future; the main clause carries it and the time clause borrows it.

This is exactly the pattern you already met in conditional sentences: if it rains, we'll stay in, never if it will rain. Time and if behave the same way in English, and once you group them together in your head you stop making the mistake in both.

Why does it feel so wrong to learners? Because most other languages allow, or demand, a future in both halves. Spanish says cuando llegue with a subjunctive, French says quand j'arriverai with a straight future. English simply took a different route. There is no logic to appeal to here. It is a habit you install by repeating correct sentences until the wrong one sounds odd.

Present simple or present perfect?

Both are allowed after a time word, and the choice is about emphasis.

When I finish work, I'll call you.
When I have finished work, I'll call you.

The present simple simply names the event. The present perfect insists that the action is completely over before the next one starts. Use it when the completeness matters:

As soon as I've eaten, we can go. (not while I am still eating)
After she's read the letter, she'll understand.
Once you've paid, the ticket is yours.

With after, as soon as and once the perfect is very common, because those words already point at a finished action. With when and while the simple is usually enough. Never use will have in the clause: when I will have finished is wrong twice over.

The time words, one by one

when - at that moment or event. The general-purpose word. When the film ends, we'll go.
while - during a longer action, with something else happening inside it. While I was cooking, the phone rang.
as - much like while, slightly more formal, often for two things changing together. As the sun went down, the air grew cold.
as soon as - the very moment something happens, with no gap. Ring me as soon as you land.
once - when something is finally done, with a sense of a stage completed. Once the paint is dry, we can move the chairs back.
until (or till in speech) - up to that time and no longer. Wait until I come back.
before / after - earlier than, later than. Take your key before you go out. We had tea after we finished.
by the time - not later than that point, usually with something already complete. By the time we arrive, the shop will have closed.
the moment / the minute - a stronger as soon as. The minute he walked in, everybody went quiet.
whenever - every time. Whenever it rains, the roof leaks.

When vs while

The two get mixed up more than any other pair, and the verb form gives you the answer.

While frames a stretch of time. It usually takes a continuous verb, and something else happens inside that stretch.

While I was walking home, I saw an old friend.

When points at a moment or a complete event. It usually takes a simple verb.

When I got home, the door was open.

Both are possible in some sentences with a real change of meaning. When she called, I was cooking puts the call in the foreground. While she was calling, I cooked makes the call the background and my cooking the news. Whichever you choose becomes the frame, and the other becomes the event.

One firm restriction: do not use while for a short, single action. While the bell rang, we stood up sounds wrong to a native ear because a bell ringing is not a stretch you can put something inside. Use when.

Until vs by

A different pair, same confusion, and both are time words so they belong here.

Until covers every minute up to a point. It goes with things that continue, and with negatives.

The shop stays open until nine.
She won't leave until the job is done.

By names a deadline: at that time or earlier. It goes with things that get finished.

Send it to me by nine.
By next year they'll have moved house.

I'll stay by six is wrong; you mean until six. I'll finish it until six is wrong; you mean by six. If the sentence is about a length of time, reach for until; if it is about a cut-off, reach for by. The future perfect lives almost entirely with by for the same reason.

Where the comma goes

A time clause can sit at the front of the sentence or at the back, and the meaning does not change. The punctuation does.

Time clause first: comma. When the rain stops, we'll go out.
Main clause first: no comma. We'll go out when the rain stops.

That is the whole rule, and it is worth knowing because it is one of the few comma rules in English that is not a matter of taste. Putting the time clause first is slightly more formal and gives it a little more weight, which is why instructions and warnings so often start with before, when or once.

The exception: when as a question word

Everything above applies to when as a time conjunction. But when also works as a question word, and then it introduces a noun clause rather than a time clause. In that job it takes will perfectly happily.

I don't know when he will arrive.
Nobody can tell us when the shop will open.
She asked me when the train would leave.

Compare the two uses side by side:

I'll phone you when he arrives. (time clause: when = at the time that)
I'll ask her when he will arrive. (noun clause: when = at what time)

That second sentence is an indirect question, which brings its own word-order rules with it.

The test is whether you can replace the clause with something or it. I don't know something works, so it is a noun clause and will is fine. I'll phone you something does not work, so it is a time clause and will is out.

Summary table

Word	Job	Example
when	at that moment or event	Call me when you get in.
while	inside a longer action	While she read, I cooked.
as soon as	the very moment something happens	We'll go as soon as you're ready.
until	up to that time and no longer	Wait until I come back.
before	earlier than that point	Eat before you go.
after	later than that point	We left after the rain stopped.
by the time	not later than that point	By the time we got there, it was shut.
once	when it is finally done	Once you've paid, it's yours.

Common mistakes

Ranked by how often learners actually make them.

Will after a time word. When I will see him, I will tell him should be when I see him, I'll tell him. This one mistake accounts for most of the errors on this page.
Will have in the time clause. By the time I will have finished is worse than the first, because it doubles the error. Say by the time I finish.
Until for a deadline. Please send it until Monday means something strange: keep sending it every day until Monday. Say by Monday.
While with a short action. While the bell rang, we stood up should be when the bell rang. While needs a stretch of time.
Missing comma when the clause comes first. When the rain stops we'll go needs a comma after stops. It costs nothing and it reads far better.
A past tense for a future event. When I finished my work, I will call you mixes the times. If the work is still to come, use the present: when I finish my work, I'll call you.

Practice

Put the verb in the right form. Answers are below.

I'll wait here until the bus ___ (come).
As soon as she ___ (get) home, she'll ring you.
While I ___ (read) the paper, someone knocked at the door.
By the time we ___ (arrive), the film will have started.
Do you know when the next train ___ (leave)?

Answers: 1. comes (present after until) 2. gets (present after as soon as) 3. was reading (a stretch of time inside which the knock happened) 4. arrive (present after by the time) 5. will leave (when is a question word here, not a time conjunction)