Thanks Doll and Paulio.
sorry didn't write down why I have doubt with these questions.
i.Those who took the courses are able to study more efficiently.
Those who took the courses are able to study more efficient.
The original sentence is the first one. I thought of the second one when I read the first one, because I thought after the 'more' should be an adjective, not an adverb.
ii.It is often possible to explain the behaviour of young people by examining the social environment where they have to live.
It is often possible to explain the behaviour of young people by examining the social environment where they live.
The former is a sentence taken from the same book. I don't understand why the author used 'they have to live', because 'have to' has the meaning of 'forced'.
iii.They made great progress in their studies and in their work
They made great progress in their studies and their work.
Same, the second one is my suggestion, while the first one is what I read from the book( and I wasn't sure if it is correct. Thanks, Paulio).
Last, what is the meaning of '
at times'? (sometimes?) <= this one I couldn't find it in my dictionary.
Just want to clearify, all the questions I posted are not school homework, although they are the problems, which I couldn't understand after I checked with the answers, taken from a grammar book.
Finally,
Hey guys, homework time. <= this one is a joke, but I doubt if anyone could get it. Really need to polish up my english.