翻译练习 -- A translation exercise
I copied the English sentence for this translation exercise from @daidengo--thank you！
1. The book I bought last winter in France is very interesting. 我去年冬天在法国买的一本书很有意思。
我去年冬天在法国买的那本书很有意思。
2. The green dress I bought with my American friend two days ago is both comfortable and beautiful. 我两天前跟我的美国朋友买的绿裙子又舒服又漂亮。
3. I drove the car for eight hours, and now I would like to sleep for at least ten hours. 我开了八个小时车，现在想睡至少十个小时觉。or 我开了八个小时车，现在想睡十个小时以上（觉）。（<--哪个句子更好？）
我开了八小时的车，现在我想睡至少十个小时的觉。so the first sentence you wrote is better.
4. They traveled by air for eight hours. 他们坐了八个小时飞机。
5. How long has she studied German? 她学了多长时间德语了？or 她学德语学了多长时间了？(Does this sentence need that 2nd 了? I wanted to give the meaning of something that is still in progress: she is still studying German right now, up to the present time.)
她学了多长时间德语了 ？or 她学德语多长时间了 ？ both are right
6. I`m tired, so I’m going to watch TV for two hours. 我很累，（因此）我想看两个小时电视。
7. We’ve talked about this complicated problem for an hour. 我们已经把这个很复杂的问题谈了一个小时。
我们已经谈论这个很复杂的问题一个小时了。or 我们已经把这个很复杂的问题谈了一个小时了。
8. I looked for the car for half an hour, but I didn’t find it. 我半个小时找了我的车，但是没找到。
我找了半个小时我的车，但是没找到。
翻译练习 -- A translation exercise
I copied the English sentence for this translation exercise from @daidengo--thank you！
1. The book I bought last winter in France is very interesting. 我去年冬天在法国买的一本书很有意思。
Great. I don't want to sound like a snob, but I really don't think that it's more suitable to use "那本书" here, because it would make me think that the listener had heard before that you bought a book last winter in France.
2. The green dress I bought with my American friend two days ago is both comfortable and beautiful. 我两天前跟我的美国朋友买的绿裙子又舒服又漂亮。
3. I drove the car for eight hours, and now I would like to sleep for at least ten hours. 我开了八个小时车，现在想睡至少十个小时觉。or 我开了八个小时车，现在想睡十个小时以上（觉）。（<--哪个句子更好？）
Both are pretty good, but I would say 我开了八个小时的车，现在想睡至少十个小时的觉。Or remove "的觉".
4. They traveled by air for eight hours. 他们坐了八个小时飞机。
5. How long has she studied German? 她学了多长时间德语了？or 她学德语学了多长时间了？(Does this sentence need that 2nd 了? I wanted to give the meaning of something that is still in progress: she is still studying German right now, up to the present time.)
Without the 2nd 了, the Chinese sentences can mean "How long has she studied German?" or "How long DID she study German?", depending on the context.
6. I`m tired, so I’m going to watch TV for two hours. 我很累，（因此）我想看两个小时电视。
7. We’ve talked about this complicated problem for an hour. 我们已经把这个很复杂的问题谈了一个小时。
Maybe you can put the object at the beginning of the sentence to avoid the difficulty you've had in constructing this sort of sentence.
这个复杂的问题我们已经谈了一个小时.
8. I looked for the car for half an hour, but I didn’t find it. 我半个小时找了我的车，但是没找到。
我找车找了半个小时, 但是没找到. or 我找了半个小时的车, 但是没找到.
