The usage of "in" in reference to the past

Question

The thing which confuses me is the usage of “in” and “for” in the following examples.

I think the following description of the meaning of these sentences is correct.

1) It has rained for the last week. (”for” represents a continuing action)
2) It has rained in the last week. (”in” represents a once-off action)

For = conveys an action lasting the whole period of time.
In = indicates that something happens within this period of time but the action doesn’t last the whole period.

My first question is:

Does “in” imply only one instance (occasion) of a certain action or more? That is, does the sentence “It has rained in the last week” say that there was only one instance when it rained or can it mean that there was more than one instance when it was raining? (I suspect that it conveys only one instance of some action, but I am not sure)

Now, the difficult part begins. I have noticed that with some verbs “in” carries the meaning of completeness. For example,

3) I have learnt English in the last 3 years. (It means that I have completed my English studies and now I know English.)

The main question:

It makes me wonder why 3) can’t be understood as “I have been involved (maybe only once) in the process of learning English in the last three years. But I haven’t mastered it.” In other words, why can’t 3) be understood as 2)? (I hope you don’t think that I mean the lexical meanings of “to rain” and “to learn”. I mean why can’t 3) mean that I learnt English only once during that period?)

I hope I managed to make my issue clear to you.

Answer 1

You ask,

I mean why can’t 3) mean that I learnt English only once during that period?

I think the problem is actually the use of the verb "learn", where you really mean "study".

The phrase "I have learnt" (or "I have learned", for my American ears) implies that your learning is complete. It uses the present perfect tense, meaning your learning is complete as of now. Saying "I have learned multiplication", for example, means you are proficient enough at multiplication that you don't need to study it any more.

I think what you are trying to say in "I learnt English in the last 3 years" is better expressed by the sentence "I have studied English in the last 3 years." The verb study better covers the meaning you're looking for: that of "learning" something one specific time, or for a period of time.

To say that you "learnt English only once" semantically is meaningless. When you say "I learnt English" it implies that you have mastered the language. You can't learn a language twice, unless you forget over the years. You can, however, study a language any number of times.

The comparison between in and for is a separate issue here. Saying, "I have learnt," or "It has rained," again means that the rain or the learning is complete. In is just a preposition that helps indicate the time frame of the rain or learning. However, when you use for here, "It has rained for the last week," the sense of completion is lost. This sentence implies "It has been raining (continuously) for the last week and it's probably still raining." (It actually changes the tense from present perfect to present perfect continuous, even though the verb doesn't change.)

Answer 2

In response to your main question, I read sentence three as meaning "At some point in the last three years, I learned English". It doesn't say the exact time you learned English, nor does it necessarily indicate you stopped studying it once you learned it (indeed, many people never stop studying it!).

As to whether or not "in" indicates exactly one occurrence of something, it doesn't necessarily. "It rained in the last week" means that at some point during that week, it rained. It could have just been Tuesday, or Monday and Wednesday. All it says for sure is that at some point in the week, it rained, then it stopped.