People have to tighten their belts (during) the recession. Which word can replace "during"?

It would be ( ) if you could help me.

Which vocabulary can only be adverb?

This is an _-friendly engine.

Which word/phrase contains the following defintion? (The state or condition of having little or no money, goods, or means of support)

When we would like to express a point which is different from the previous thing we have mentioned, we would use the conjunction "_".