I would like to know whether the sentence "I studied Chinese in one month" is correct. If yes, what's the difference then if compared to "I studied Chinese for one month"?
I learnt English in 3 months.
I learnt English for 3 months.
If you say that you learned English in one month, then you learned the whole language during that time, but if you say you learned it for a month, then you are just describing the length of time spent studying, without any implication of how much you learned.