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1

This question already has an answer here:

Should I say:

When I am 18, I'll take my driving test

or

When I'll be 18, I'll take my driving test

Which one is the correct sentence?

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This question has been asked before, many times over. Please search the site before posting. – RegDwighт♦ Feb 28 at 11:19

marked as duplicate by RegDwighт♦ Feb 28 at 11:16

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

5 Answers

up vote 16 down vote

If 'when' is used to indicate a context in the future, the 'will' becomes redundant and 'when' is followed by the present form of whatever verb.

In this case: when + subject + present simple , subject + future simple

When I know the answer, I'll call you

When he leaves for work, I'll telephone.

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I think this is the best explanation of why we don't use will in the OP's sentence. – LarsH Feb 27 at 16:54
Note that (as LarsH mentions below) this depends on the type of clause; for example, we say "I don't know when I'll know the answer", not *"I don't know when I know the answer." (In traditional grammar terms, the difference is between an "adverb clause" and a "noun clause". In modern terms, I think the terms are "adjunct" and "content clause".) – ruakh Feb 27 at 23:21
up vote 8 down vote

Of the two sentences asked about, the first is correct and the second is incorrect.

You can also say "When I turn 18, I'll take my driving test".

You don't need "will" in both clauses, only the main clause (the second one in all these cases).

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up vote 5 down vote

English has no future tense, but can express the future in several different ways. Will (or, in speech, ’ll) + the plain form of the main verb is one of them and is typically used to make a prediction or to express a decision about the immediate a future. For this reason, it is clearly not appropriate in your example.

Another way in which English expresses the future is by using the present tense, and this is typically used to talk about future certainties. In the example, the speaker thinks that being 18 one day is a certainty, and that is why the present tense is used.

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2  
Does the criterion of making a prediction or expressing a decision fully explain why we don't use will in the OP's sentence? We do use it in sentences like "I don't know when he'll be 18" or "The sign doesn't say when the store will be open." Do those fit the "prediction" criterion better than the OP's sentence? – LarsH Feb 27 at 16:53
It’s a good question, which could take up more space than is available here. The best I can do is to say that there is normally no need for those who are, say, 15, to predict that they will be 18. We can assume that they will live another 3 years, and there’s no element of prediction involved. That’s what makes When I'll be 18 inappropriate. The negative introductions to your examples, on the other hand, introduce some doubt over the timing, which permits the use of ’ll. – Barrie England Feb 27 at 17:47
It at least explains why it's jarring to some native speakers. That was my initial reaction to the question as well. "It makes it sound like he or she is suffering from some disease and is not sure yet if they will live to be 18." – Affe Feb 27 at 18:47
Prediction seems to fit pretty well, but I'm not so sure about uncertainty. You see, I do know when he'll be 18, because I read his birth certificate. Then there's "you will know when he is 18" (because when it happens he won't be able to stop talking about it), and "you will know when he'll be 18" (because I'll tell you later). I think that the distinction is whether the phrase is the object of discussion, or merely providing supporting context. – Jander Feb 28 at 6:13
up vote 4 down vote

When I am 18, I'll take my driving test

would be the correct sentence structure

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Could you expound on why this is correct? – Luke Feb 27 at 15:20
up vote 0 down vote

Although both clauses imply future tense, "will" is not used in the clause starting by "when". Then the first sentence is correct, the second is not.

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