He went to school before he did his homework.這個主詞可以省略嗎??

主詞可省略，寫法有二種：

Before doing his homework,he went to school.

He went to school before doing his homework.

應該不行把"主詞"省略.附屬子句的主詞在意義明顯之下,就通常會被省略.

He (主詞) went to school before he (附屬子句的主詞,而非"主詞")did his homework. (OK)
went to school before he did his homework. (X)
He went to school before doing (his) homework. (O)

He had done his Homework before he went to School.----The past perfect tense !
He had not done his homework before he went to school-----The original meaning declared;
Omitting "he"=omitting "he did"; did=>doing=past participle:-
Before "doing" his homework , he went to school.---(omitting two to become one subject=he)
Or:- He went to school before "doing" his homework.---The psychological moment is not appropriate when we see "he"placed at beginning is most likely to achieve NOT the desired effect of one subject declared.Hence,the first one preferrable.
The past perfect tense without the subject:; (an attempt to answer only)
eg:-Before the homework was being done, the "went to school" Buz had been set up by a subject "he".

As soon as finished homework at home, he went to school as usual. Yip