Consider:
a. I have learned English for 6 months.
b. I have learned English in 6 months.
Are the two sentences above grammatically correct?
Do they carry the same meaning?
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a. I have learned English for 6 months.
b. I have learned English in 6 months.
Are the two sentences above grammatically correct?
Do they carry the same meaning?
Learn is what linguists call a 'telic' verb: one which in its basic use implies that its action moves toward and achieves a final goal, a 'change of state'. In the case of learn the 'change of state' is from ignorance to knowledge.
I have learned English for 6 months.
The construction for TIMESPAN is typically used with a 'non-telic' verb: one which in its basic use designates an activity which does not have a final 'goal' but continues indefinitely: I have lived here for six months. When a for TIMESPAN is used with a telic verb it usually signifies that the action was carried out repeatedly. For instance, She has written novels for ten years is taken to mean that she wrote a novel, and then wrote another novel, and then wrote . . . and so forth.
That is probably not what you mean. You may have learned a great deal of English, but you have not learned the entire language. (Neither has anyone else, including me!) I suspect what you want to say is that you have studied English for six months—study is a non-telic verb signifying the activity of attempting to acquire knowledge, not successfully completing the attempt.
I have learned English in 6 months.
In TIMESPAN is something quite different: it designates the time it took you to complete an action, and is used only with telic verbs. You might for instance say In six months I have learned enough English to ask intelligent questions. As before, however, I have learned English without any qualification implies that you have mastered the entire language; it would be quite extraordinary to achieve that in six months, or even six decades!
for indicates time up to now according to its usage. but we can't see it with in.we can tell I was a student in 1995. if it gives a same meaning we can use I was a student for 1995. I think every thing has a usage.