Learn English – Which one is correct, “best wishes to you” or “best wishes for you”

Correct is an adjective and correctly is an adverb. In Standard English, but not necessarily in other varieties of the language, verbs are modified by adverbs, so You heard it correctly is grammatical in Standard English and You heard it correct is not.

• 1.) Which one is you?

• 2.) Which one are you?

Which is correct?

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Both are "correct". They just have different subjects.

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LONG ANSWER VERSION: Let's identify the subject of each interrogative clause, by using the verb's number as the indicator:

• 1.a) Which one is you?

• 2.a) Which one are you?

Notice that there is formal subject-verb agreement between the subject and verb in each version.

Let's verify that we've actually identified the subject, by looking at a related declarative clause for each of those above two interrogative clauses.

For #1.a. "Which one is you?":

• 1.b) That one is you. -- [declarative clause]
• 1.a) Which one is you? -- [interrogative clause]

The conversion from declarative #1.b to the interrogative #1.a was a straightforward change from "That one" to "Which one". The subject of both versions are before the verb.

For #2.a. "Which one are you?":

• 2.b) You are that one.
• 2.c) You are which one? -- [interrogative clause, interrogative phrase in situ]
• 2.a) Which one are you? -- [interrogative clause, fronted interrogative phrase]

Notice that when the interrogative phrase ("which one") is fronted in a main clause, that causes obligatory subject-auxiliary verb inversion. That is what happened between the last two steps.

To verify this, we can use a multi-word verb phrase to see this subject-auxilary verb inversion working:

• 3.a) You would be that one.

• 3.b) You would be which one?

• 3.c) Which one would you be?

Notice how the subject "you" ended up getting sandwiched between "would - be".

A similar exercise can be done with the subject "Which one", except there is no subject-auxiliary verb inversion because the interrogative phrase is the subject:

• 4.a) That one would be you.

• 4.b) Which one would be you?

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Short story: One way to find the subject of an interrogative main clause is to convert the example sentence into one that uses a multi-word verb phrase. The subject will either be in the front before the verb phrase, or else it will be sandwiched in between the verbs.

Using the OP's two examples:

• 1.) Which one is you?

• 2.) Which one are you?

we see that the versions can be converted into the below:

• 1.x) Which one will be you?

• 2.x) Which one will you be?

Since the OP's two examples only had a single verb in them ("is" vs "are"), then as to what the subject is will, in this case, depend on the verb that is used -- for the speaker/writer will have used subject-verb agreement between the subject and verb.

So, the answer is: Both are "correct". They just have different subjects.