What verb to use after "did"?

I could not for the life of me understand what verb to use after "did".

All I know is that the verb that follows "did" should be used in its base form. Example: Did you “eat” your lunch this afternoon?

However, how come it is “I didn't think it WAS possible, but not “I didn't think it IS possible?”

What if we have multiple subjects performing different actions following “did”?

Example: I didn’t “go” to the market and “write” the letter.

  • 1
    In your example, the verb following auxiliary "do" follows the rule and hence is the infinitival (base form) "think". But the verb "be" is part of a separate verb phrase located in a subordinate clause "it is / was possible", where "was" is past tense, and "is" present tense. In "I didn’t [go to the market] and [write the letter]", there are two coordinated verb phrases (bracketed), each subject to the rule that auxiliary "do" requires an infinitival verb phrase as complement.
    – BillJ
    Apr 19, 2017 at 8:05

1 Answer 1


You're mixing something up here: the infinite verb in your second example is think, not was:

I didn't think it was possible

The general rule is: English questions and negations in the simple and past tense typically need an inflected form of the verb do plus a main verb in the infinitive:

I don't like grammar.
Do you like grammar?
He didn't find it.
Did he find it?

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