When a sentence about the future contains two clauses — a main clause and a time clause or conditional clause — there is one rule that students consistently break: you cannot use will in the subordinate clause.
The subordinate clause is the one introduced by a conjunction like when, as soon as, before, after, until, once, by the time, while, if, unless. Even though the meaning is clearly future, the verb in this clause must be in the Present Simple — not will. The will (or won't) goes only in the main clause.
All of these conjunctions introduce a time clause. None of them can be followed by will:
| Conjunction | Meaning | Example |
|---|---|---|
| when | at the time that | I'll call you when I arrive. |
| as soon as | immediately after | I'll text you as soon as I know. |
| before | earlier than | Do this before you leave. |
| after | later than | I'll relax after I finish. |
| until / till | up to the point when | I'll wait until you come back. |
| once | when / as soon as (completed) | Once you try it, you'll love it. |
| by the time | before the point when | By the time she arrives, we'll be gone. |
| while | during the period when | While you are away, I'll look after it. |
The same rule applies to conditional clauses introduced by if and unless. The condition clause uses the Present Simple; the result clause uses will.
The time or condition clause can come first or second in the sentence. The rule about which clause gets will does not change — only the punctuation changes (a comma is used when the subordinate clause comes first):
✗ I'll call you when I will arrive. · As soon as she will finish, I'll review it. · By the time we will get there, it'll be over.
✓ I'll call you when I arrive. · As soon as she finishes, I'll review it. · By the time we get there, it'll be over.
No matter how clearly the time clause refers to the future, will is never used inside a clause introduced by when, as soon as, before, after, until, once, by the time, or while. The clause is understood to be future from context — the Present Simple handles it. Think of it as: the conjunction does the future work; the verb just uses the Present Simple.
✗ If it will rain, we'll cancel. · Unless you won't study, you'll fail. · If she won't agree, we'll find someone else.
✓ If it rains, we'll cancel. · Unless you study, you'll fail. · If she doesn't agree, we'll find someone else.
The condition clause (after if or unless) always uses the Present Simple in first conditional sentences. Never use will or won't in the clause after if or unless. A special trap: unless already means if not — so unless you won't creates a double negative that is always wrong. Unless you study = if you don't study.
✗ I call you when I arrive. · She tells you as soon as she knows. · If it rains, we cancel.
✓ I'll call you when I arrive. · She'll tell you as soon as she knows. · If it rains, we'll cancel.
The main clause — the clause that contains the result or the main action — must use will (or another modal) when the sentence refers to the future. The Present Simple in the main clause makes the sentence sound like a general fact or habit, not a future action. The pattern is: Present Simple in the subordinate clause, will in the main clause — not the other way around, and not Present Simple in both.
✗ Unless you won't call me, I'll worry. · Unless she won't agree, we'll proceed. · Unless it won't rain, we'll go out.
✓ Unless you call me, I'll worry. (= If you don't call me, I'll worry.)
✓ Unless she agrees, we won't proceed. (= If she doesn't agree, we won't proceed.)
✓ Unless it rains, we'll go out. (= If it doesn't rain, we'll go out.)
Unless already contains the idea of negation — it means if not. Adding won't or not inside an unless clause creates an unwanted double negative that reverses the meaning. Unless is always followed by a positive Present Simple verb: unless you call, unless she agrees, unless it rains. Never: unless you won't call, unless she won't agree.