Learn English – the subject of “How to do?”

Your sentence is grammatical as it stands, and having a single subject with two conjoined verb phrases is common in all forms of writing and understandable in general.

However, the problem with easily parsing your particular sentence stems from the fact that results can be both a 3rd person singular form of a verb (as you intend) or the plural form of the noun result. Because of the way the sentence is constructed, both interpretations of results are possible until you reach the end of the sentence and see that noun-results would have no verb in its clause. At this point, the reader must go back and try to parse the sentence differently.

Using it in front of results is not a grammatical requirement, but it eliminates this ambiguity. Other similar ways to avoid this ambiguity include:

• "In both cases, execution is asynchronous, which results in the execution..."
• "In both cases, execution is asynchronous, resulting in the execution..."

Both sides of this question have been argued in the linguistic literature. "who" could be a topic, and then the sentence structure would be

[who [ __ hears a noise]]

on the analogy of other wh-questions with a wh-word moved to the top of the structure and leaving a gap, __, where it once was. Or, perhaps questions whose subject is a wh-word simply don't need to be changed by moving the wh-word to the front, because, well, it's already there. In that case, we have

[who hears a noise]

and "who" is simply a subject.

There are a number of grammatical theories which do not permit stating grammatical relations in terms of word order, but only structural relationships, so such theories would presumably not recognize the logic of the argument that a wh-word subject must remain a subject in a question, because it is already at the front of the sentence, where it needs to be. (Such "order free" theories are Chomsky's latest theories (I think), dependency grammar, relational grammar, and GPSG.)

You might think that the subject-agreement in the verb "hears" with the singular "who" shows that "who" is a subject. And perhaps that is evidence, but a follower of the east coast school of linguistics would assume that the gap created by extracting the subject is a "trace" which is coindexed with the former subject, so the verb agreement can still be correctly described.

There is an argument for the second no-movement treatment quoted and discussed in the book Generalized Phrase Structure Grammar, and the argument itself (if I recall correctly, due to Pauline Jacobson) is based on so-called parasitic gaps. In the Wikipedia entry for Parasitic gap, this illustration is given:

Which explanation did you reject __1 without first really considering __2?

where the first gap must be higher up in the structure tree than the second (parasitic) gap. The relevance of parasitic gaps here is that they give us a diagnostic for detecting a gap in subject position. A subject gap, since it is highest in the structure of its clause, should license a parasitic gap elsewhere in the clause. But if there is no subject gap, there won't be any parasitic gap, because parasitic gaps depend for their existence on a gap higher up.

So, we can construct a test case from the above parasitic gap sentence by making it a passive whose subject is the wh-expression:

*Which explanation __1 was rejected by you without first really considering __2?

This is ungrammatical. If there were really a subject gap, it should have been okay. So, we can conclude that there is no subject gap and that a wh-subject in a question remains in place. I know of no evidence on the other side of this question, so that is my conclusion.