Question: There's the following sentence in my Grammar book: 

> 
> It was a lovely surprise to find that all the washing-up **HAD BEEN DONE** while I was asleep.
> 
> 
> 

The only answer mentioned in the answer key is past perfect (in capitals). Since today, I've thought that we use past perfect to clarify the order of actions but if it's clear by itself we don't need one. 

I feel, in this sentence the order of actions is obvious, so **WAS DONE** is also possible. Even more—the past perfect isn't necessary. 

Would someone please explain which tense should be used and why?

Answer: 
"by a month" is simply not idiomatic at all in English here. 

**For time**, one does things or things are done **in a month, week, year, day, hour etc**. That is the period of time that will elapse.

**He beat the deadline by a month**.

There, **by a month is used to measure the number of months he won by.**

Some project has to be handed in in six months. The guy hands the project in in five months. He has beat the deadline ***by a month***.

For time that will elapse (go by) we use in: in a month.

To measure some amount of time in relation to a set time, we use by:

He beat me by ten minutes. They beat us (in the sailing race) by a month.
"In a month" is correct. It means that it will take 1 month by the time he'll speak fluent French. "By" is used to indicate the end point of an event. For example, you can say "He'll speak fluent French by February". February is the deadline by which the activity will be completed.