Learn English – Which one is a correct sentence ‘classes has started’ or ‘classes have started’

The OP's question involves the topic of interrogative pronouns (e.g. "who" and "what") and the question of whether they could be considered to be singular or plural.

In general, the interrogative pronoun "who" takes the default value of singular; and when it does take the value of singular, its question can allow both singular and plural answers. And so, that's why we often see questions using the default singular (that is, singular subject-verb agreement).

But when the speaker is expecting only a plural answer, then the speaker could use a plural override when forming their question -- but usually that is not obligatory, and usually the speaker can still use the default singular in their question. But, sometimes there are exceptions, when the plural override is obligatory, and that obligatory plural override often occurs when the grammar of the question requires or strongly prefers a plural verb (e.g. a predicative complement realized by a plural noun phrase).

So, with that as a grammatical background, let's look at the OP's context for those candidate questions. The OP's context is described as:

The situation is that: We've already read three English novels.

where I'm assuming that two or more of the addressees have read at least 3 novels. It is possible that the "we" includes only the speaker and one other person, but I'd think it more feasible that there are three or more people involved here (e.g. a classroom with more than one teacher and two students). The rest of my post will work with that assumption: there is a speaker and at least two addressees.

If the speaker doesn't really know whether the answer will be singular or plural, or is willing to expect either a singular or plural answer, then the speaker will (in general) use the default singular for their question:

1) Who has already read three English novels?

because that allows both a singular and a plural answer.

But if the speaker expects that more than two (students) will respond in the affirmative, then, often the speaker might choose to use the optional plural override:

2) Who have already read three English novels?

because the speaker is expecting to get two or more affirmative answers. But, this is optional to the speaker; the speaker can still use the default singular version, because this is not one of the exceptions that I had mentioned earlier.

ANSWER: So, to answer the OP's questions:

Which is the correct question for that situation? Or could both be correct?

for the specific candidate questions, it seems that both versions could be considered to be grammatical for that given context. It would be up to the speaker as to which one they would want to use.

= = = = = SOME VETTED GRAMMATICAL INFO = = = = =

OP asked:

Please also provide the grammatical reasons for your answers.

For more information on this topic, there is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language (CGEL), pages 505-6.

Related excerpt from CGEL pages 505-6:

(d) Interrogatives

In general, the interrogative pronouns who and what take the default value of singular. Compare:

[19]

• i.a. Who wants some more ice-cream? - - - b. What remains to be done?

• ii. Which (of these) is/are yours?

There is no presupposition in [i] that only one person wants some more ice-cream or that only one thing remains to be done: the default singular allows for either singular or plural answers. In [ii], with determinative which as fused determiner-head, we have a singular or plural verb according to whether the answer is presupposed to be singular or plural.

The default singular values for who and what can, however, be overridden when there is a presupposition that the answer is plural:

[20]

• i. What are going to be the deciding factors?

• ii. Who haven't yet handed in their assignments?

• iii. Who have excelled themselves in this year's coxed pairs?

• iv. What have pointed ears and long tails?

In [i] the override is obligatory: this case is similar to those discussed for fused relatives such as [18.i], with the plural PC the deciding factors forcing a plural construal of what.

A likely context for [20.ii] is one where I'm addressing a group of students and assuming that a plurality of them haven't handed in their assignments; singular hasn't would be possible (but without indicating any expectation of a plural answer and favouring singular assignment if there is only one each).

In [20.iii], coxed pairs involve three people (two rowers and the cox), so the presupposition is again that the answer is plural. The reflexive has to be plural, and this favours a plural verb.

Finally, [iv] presupposes a generic bare plural as answer, e.g. foxes, but the motivation for a plural override is relatively small since the answer could be given in the form of a generic singular, e.g. a fox.

There's also some related information in the older 1985 reference grammar by Quirk, Greenbaum, Leech, Svartvik, A Comprehensive Grammar of the English Language (1985 Quirk et al.). On page 371, footnote [a]:

[a] Who has both singular and plural reference, but when neither is explicit in the linguistic context, singular concord is the unmarked term. Thus, even though several voices are heard outside, the natural question will be Who's there? rather than ?* Who're there?

Note that the ?* symbol means: tending to unacceptability, but not fully unacceptable. Also, on page 756, footnote [a]:

[a] . . . Similarly, interrogative who and what as subjects normally take a singular verb even when the speaker has reason to believe that more than one person or entity is involved: Who is making all that noise? However, a plural verb may be used if other words in the sentence indicate that a plural subject is expected in the answer (Who have not received their passes?). . . .

EDITED: As a commenter has mentioned, there are also echo questions, where the "who" question can easily use a plural verb. For example:

• A: "That gorgeous blonde girl that just moved in across the street, and the redhead that you're too shy to talk to, and also that girl who's always trying to beat you up on the playground, they are coming to your birthday party."

• B: "Who are coming to my party?"

Or perhaps the simpler example as given by the commentator:

• A: "They have done it."

• B: "Who have done it?"

Here's an example borrowed from a comment in a related thread, where a plural verb might be obligatory:

• "Who haven't faced each other in the competition yet?"

Though you likely know that grammatical rules are often violated, subject/verb agreement with either/or and neither/nor generally follows the rule below.

The verb in an either/or statement agrees with the noun that is closest to it.

So either of the following would be correct (though the first one sounds vastly better to my AmE ear.)

Neither he nor his servants have left.
Neither his servants nor he has left.