lets talk about the pattern
main clause +comm doing
in this pattern, comma doing can stand before or after the main clause
leaning gmat well, I get into Havard.
I learn gmat, making my english better.
the point here is that only some but not all conjunction can stand before doing to add some meaning
instead of leaning english, I go out for change
as well as leaning english, I leanr French.
in the above pattern, doing refers to the subject and modifies the whole clause. there are many , but not all, meaning relation between doing and the main clause.
because of learning hard, I pass gmat.
this is wrong because "because of" can not stand doing when doing refer to the subject -when doing is participle-.
" as well as" normally connect 2 similar things. I learn english as well as french.
but those phrases can stand before doing. doing in this case is participle, which has some, but not all, fuctions of adjective and verb. I dont know whether these phrase are conjunction or preposition when they stand before doing. but the key point is they can stand before doing this way
doing could be a pure noun , a participle or a gerund. pure noun can go with article such as "a, the" and adjective. participle has some, but not all , functions of adjective and has some but not all, functions of verb. gerund has some, but not all , function of noun and has some but not all, functions of verb.
doing is basic grammar point and gmat test this point a lot. but the problem is that most grammar books do not explain doing well and, non native test take fail on many SC problems.
many basic and hard points gmat tests because they are basic. but these points is not explained well in grammar books. for example, present perfect is not well explained in most grammar books though this point is basic. we come to this forum just to see the explanation from successful test takers who , though successful, are native to english and, so, can not give full explanation of those basic point of grammar. worst, the non natives read grammar books and focus on hard and not-basic points of grammar.