Quote Originally Posted by Antonio1986 View Post
My simple conclusions are:
1. за + ... = In + time period
2. на + ... = For + time period
1. We opened two new factories in / during three years = Мы закрыли два новых завода за три года. - Good
2. They have issued the new driving licence in six days = Выдали новые водительские права за шесть дней. - Good
3. He sold his soul to the devil for 100 years = Он продал свою душу дьяволу на сто лет. - Good
4. Can you imagine your self after 10 years = Можете представить себя ЧЕРЕЗ десять лет.
5. I will finish my research in 30 days = Я закончу свое исследование за тридцать дней . - Good. However, this will be true if you estimate the TOTAL you need to finish your research. BUT, if you say when the research will be finished, for example, you've been working on it for some time but you say when it's finished you have to say:
Я закончу своё исследование через тридцать дней.

6. After the accident he couldn't walk for 5 years = После аварии, он НЕ мог ходить [B пять лет[/B]. You don't need any preposition before time expression here
7. The deficit will be covered in 3 years = Дефицит покроется за три года. - Good
8. He spent three days alive in the cold see, after his boat was sunk = Он провел три дня в холодном море, после того как его лодка затонула. - No need to use жив in there.
9. Soviet Union lasted for 69 years = Союз Советских Социалистических Республик просуществовал шестьдесят девять лет - No preposition needed. And длился is not good in this case
10. During the last five days I slept less than 10 hours = За последние пять дней я спал менее десяти часов or В течение пяти дней я спал менее чем десять часов or Во время пять дней я спал менее десяти часов - The first variant is just great and very natural, the second one is not that good still acceptable, although it doesn't directly say that you didn't sleep well the last five days, maybe those 5 days were 5 days from some other time in the past. The third one is not good at all.