In the first sentence that starts off with a verb and ask a question "Did you do [that]?" I only understand that this question has [that] which stands alone from a noun and it is singular. The second person {you] is singular because the distributive pronoun [that] is singular. Are the verbs [Did] and [do] plural or singular? If the two verbs are singular than it fits the general rule. But if the verbs are plural than they don't fit the general rule above. Why is that please?
2 Answers
As with your other question, you seem to be very confused about noun-verb agreement.
The noun which the verb must agree with is the subject, which in your sentence is "you." As it is an interrogative sentence the verb comes first, so "did" is the main verb which must agree with the subject "you."
The bare infinitive "do" does not have to agree with any noun in the sentence, and its object "that" has no bearing whatsoever on either the subject or the main verb:
Did you do that?
Did they do that?
Did they do those things?
Did she bake them?
Did she bake it?
Do you do that?
Do they do that?
Do they do those things?
Does she bake them?
Does she bake it?
The only part of the sentence that changes to agree with any other part is the main verb, which must agree with the subject. Whether the object of "to do" is singular or plural makes no difference.
The main verb is "did". It is past tense. It should agree with the subject, which is "you".
But the past tense verb "did" has this pattern in questions:
Did I do
Did you do
Did it do
Did we do
Did you do
Did they do
Which is easy to remember! The object "that" is completely irrelevant
The verb agrees with the subject. But for this past tense verb the agreement is very simple.
Note. Verbs don't have "singular" and "plural".
