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I learnt at school that you use the present tense in an adverb clause even if you mention the future, but I often encounter adverb clauses with the present-perfect tense. I would like to know the difference between the two.

  • 1-1. I'm going to Rome when I have finished my Italian lessons.
  • 1-2. I'll go shopping if it has stopped raining by this afternoon.
  • 1-3. Don't drive a car when you haven't had enough sleep.

These three sentences are from my English textbook at school. What is the difference between the sentences above and below?

  • 2-1. I'm going to Rome when I finish my Italian lessons.
  • 2-2. I'll go shopping if it stops raining by this afternoon.
  • 2-3. Don't drive a car when you don't have enough sleep.

My textbook doesn't describe the difference. I'd appreciate it if you could help.

2 Answers 2

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The sentences in (1) imply a state - you will be able to speak reasonably good Italian, the weather will be fine, you are tired because you haven't had enough sleep.

(2-1) and (2-2) are possible (you will set off for Rome as soon as the lessons finish, you will go out as soon as the rain stops).

(2-3) doesn't really work, because you are talking about how well you slept the previous night. You could say Don't drive a car when you are not getting enough sleep, meaning that most of your nights are sleepless.

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Functionally, the first two sentences in groups 1 and 2 are identical. There's no reason I can think of why someone would choose, say, 1-1 over 2-1 or the other way around. If I said those two sentences one after the other, a native speaker might not notice they were two different sentences.

With 1-3 and 2-3, however, there's a slight difference, which is because of the meaning of "have sleep". In that context, "have sleep" is an action verb meaning something like "do sleep", rather than indicating possession of a quantity of sleep. So in the simple present, which has the meaning of doing things in general, sentence 2-3 means something like:

Don't drive a car when you don't get enough regular sleep in general.

This doesn't mean the same thing as:

Don't drive a car when you are sleepy

which is the intent of 1-3.

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