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English grammar books say that when you express a period of time in present perfect, the prepostions such as "for" or "since" should be used.

Example sentences:

I have lived here for 20 years.

I want to do something I haven't done for years.

I have studied French for three years.

Can I use "in" instead of "for" in these sentences? Are they interchangeable?

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2 Answers

In can be used instead of for only in the second example. The other sentences can have only for to be correct.

Edit upon comment: In can be used to show the same thing after first, last, etc as in:

It's the first email I've had in ten days.

And as it's been pointed out, when the sentence is negative, in and for are interchangeable.

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However, the negatives may be OK: I haven't lived here in 20 years ... I haven't studied French in 3 years. ... Probably "for" is still better, but "in" is OK. The reason the second is OK is that it is negative. I want to do something I have done in years is not OK. –  GEdgar Nov 20 '11 at 18:13
    
Correct point, I had never noticed. Thanks. –  Irene Nov 20 '11 at 18:23
    
Can you elaborate on why this is? –  Nate Eldredge Nov 20 '11 at 18:32
    
@Nate Eldredge: Look at the edits in my answer. I looked the information up when I saw GEdgar's comment. It's listed in the uses of the preposition "in". –  Irene Nov 20 '11 at 18:51
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Here are some thoughts on the topic from a screenplay perspective.

As is pointed out in the comments,

If one uses "for" in temporal context, the use of for is necessary when referring to a positive span of time. As in:

I smoked for five years.

If one uses for in a negative temporal context:

I have not touched a cigarette for five years.

it seems to carry a slight tinge of continuation of the state, compare this to

I have not touched a cigarette in five years.

which seems to bring that state to current close. That is to say, the in implies a closure of the period.

if i was to write a dialogue in a screenplay for two situations:

someone who is saying no to a cigarette i would use the first.

someone who is accepting a cigarette (possibly in a nervous situation and is talking to themselves) i would use the second.

Addendum

Compare the following remarks from one person to another in a house:

I haven't been here for a long time.

vs.

I haven't been here in a long time.

The former implies time period that is continuing (starting from arrival) whereas the latter refers to the state that ended when the subject arrived at the house.

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