I was confused by Grammarly which keeps saying that I have to use is instead of are in the example below.
Such are his usual responses to every question he is being asked.
Can someone clarify which form of to be I should use and why?
Grammarly, in its limited way, is trying to make the verb agree with "such" without looking at the referent, "usual responses". By itself, the number of "such" is ambiguous. Both of the following statements are grammatically correct.
Such is his usual response to every question he is being asked.
and,
Such are his usual responses to every question he is being asked.
When faced with an ambiguous pronoun like "such", "which", and "who", you have to rely on the referent to indicate number. In your case, "usual responses" indicates that "such" is plural; in the added example I provided, "usual response" indicates that "such" is singular.
In your example sentence, to be is referring to responses. Therefore, because responses is plural, so is to be.
One way to look at this is by rephrasing the question. In English, the grammar will rarely (if ever) change just by changing the structure of the sentence.
So if we rephrase this sentence, which is correct?
His usual responses [to every question he is being asked] is such
OR
His usual responses [to every question he is being asked] are such
Now it's more intuitive that are is the correct choice here.