I sat down with a student named Forrest today to go over the use of prepositions in time phrases. Consider the following sentences:

I lived in Paris for a month.
I came to Paris for a month.

In the first sentence ‘for a month’ is translated «месяц», and in the second it's «на месяц».

I'll fly to Paris in a week.
It was a slow train; we got to Paris in a week.

In the first sentence ‘in a week’ is translated «через неделю», and in the second it's «за неделю».

That's just the tip of the iceberg. Prepositions in time phrases are completely insane, and it takes a lot of practice to get them right. Of course, English is not much better. Consider:

I was there last week.
I was there on Monday.
I was there in August.

In the first sentence you can't say ‘in last week’ or ‘on last week’; you must not use a preposition. In the second sentence you can leave out the preposition, but you can't say ‘in Monday.’ In the third you can't leave the preposition out, and you can't say ‘on August.’

Yup, languages take a bit of work.