So according to rule we should use bare infinitive after modals and auxiliary verb do, so my question is that, is this sentence grammatically correct

What he did was wrong.

Or it should be

What he did is wrong.

I am confused with this sentence, i mean “was" is not a bare infinitive, is it grammatically correct to use past tense after do, did and does.

  • But "did" is not an auxiliary in your example but a lexical verb, so it does not require a following infinitival clause .
    – BillJ
    Commented Jul 23, 2019 at 17:23
  • @BillJ And how do i identify whether "did" is used a auxiliary or lexical verb. I am a beginner and English is my 3rd language so it's tough for me to understand.
    – user98725
    Commented Jul 23, 2019 at 18:06
  • 2
    The auxiliary "do" is a 'dummy' verb that occurs in three kinds of construction: (1) negation as in "I do not like fish", (2) interrogatives as in "Do you like fish?", and (3) emphasis as in "But I DO love you" / "Ed never DID like Rosie".
    – BillJ
    Commented Jul 24, 2019 at 6:06

2 Answers 2


Both phrases are correct, and "using bare infinitive after modals and auxiliary verbs" are needed.

What he did refers to an action, as in kill a person. So, both your phrases can be seen as:

(ACTION) was wrong. / (ACTION) is wrong.

Therefore, choosing between was and is depends on the context, like so:

Killing a person is wrong!

Killing a person was wrong of him.

He killed a person yesterday, and that (what he did) was wrong of him.

And so on...

  • 1
    You need to make it clear that "did" is not here an auxiliary verb, but a lexical one.
    – BillJ
    Commented Jul 23, 2019 at 17:27
  • @krobelusmeetsyndra To avoid misleading a learner, you should have checked your grammar before you posted your answer. Do you see what's ungrammatical about "Both phrases are correct, and 'using bare infinitive after modals and auxiliary verbs' are needed"?
    – Apollyon
    Commented Jul 30, 2019 at 6:48
  • And you need to point out that modals are a subset of auxiliary verbs in English, so it makes no more sense to say "modals and auxiliary verbs" together than to say "huskies and dogs."
    – Apollyon
    Commented Jul 30, 2019 at 6:53

You're interpreting "after" in too narrow a sense. It is true that when "did" (or other parts of "do") is functioning as an auxiliary within a VP (verb phrase), then it must be followed by a bare infinitive, eg "Did you see it?" or emphatic "I did see it!".

But in analysing language, it is important to work not just from the sequence of words, but from how they go together. Your example parses as

[What he did] was wrong.

"Did was" is not a consituent of this sentence in any way, but two independent verbs which happen to follow in sequence.

"What he did" is a fused relative clause, functioning as subject. In it, "did" is a full verb, not an auxiliary.

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