I would say "I learned to drive in 6 months" emphasize how long it took to learn to drive & "I learned to drive for 6 months" emphasize how long it lasted and the continuity of the action.

Here are some of my studies:

In some dictionaries, they say

You use in to say how long something takes.

He learned to drive in six months.

The food was all eaten in a few minutes.

Be Careful!

Don't say that something lasts or continues 'in' a period of time. Say that it lasts or continues for that time.

I have known you for a long time.

I worked for the same company for ten years.

But in other dictionaries, they see "in" as "after a particular length of time" not "how long it takes"

in: after a particular length of time

to return in a few minutes/hours/days/months.

It will be ready in a week's time (= one week from now).

She learnt to drive in three weeks (= after three weeks she could drive).

But other dictionaries distinguish "in" when to say when something will happen & when to say how long someone takes to do something

We use in to say how long it takes someone to do something:

He was such a clever musician. He could learn a song in about five minutes.

We use an apostrophe -s construction (in a year’s time, in two months’ time) to say when something will happen. We don’t use it to say how long someone takes to do something:

I won’t say goodbye because we’ll be seeing each other again in three days’ time. We can also say in three days, without time, in this example.

He ran the marathon in six hours and 20 minutes.

  • 1
    Excellent research on the meaning and usage of in, but what happened with for? – Mari-Lou A Oct 1 '17 at 8:47
  • I think the contrast is clearer between "I learned German in 6 days" and "I learned German for 6 days". In the first case, it seems I am clever and German is easy, and now I can speak German. In the second, I went to classes but perhaps I am too stupid and German is too difficult so I stopped – Henry Oct 1 '17 at 8:51
  • @Mari-LouA, "for" is so obvious. That is it expresses how long something lasts. That is why I don't mention "for" a lot! – Tom Oct 1 '17 at 8:52
  • They're such different ideas, they shouldn't be compared. I learned to drive for six months is simply inappropriate. If you want to put it that way, you need I studied driving for six months – Robbie Goodwin Oct 2 '17 at 18:31
  1. I learned to drive in six months

It took the speaker six months to learn "how" to drive.
Result: Today, the speaker knows how to drive, and I would understand that he or she passed their driver's test.

  1. I learned to drive for six months

At a specific moment in the past, someone taught the speaker how to drive for six months but for an undisclosed reason, the speaker either stopped learning or stopped driving.
Result: The sentence implies that the speaker does not drive today or stopped learning to drive.

  • 1
    Yes. The essential difference between using "in" and using "for" in this context is that "in" suggests the thing has been completed. I am not sure it always means "satisfactorily completed", otherwise why would we say "His ability at playing tennis is that of someone who learned in a weekend". – WS2 Oct 1 '17 at 9:22

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