For example:

I have not booked the flight.
I did not book the flight.

I haven't bought the lemons.
I did not buy the lemons.

  • 2
    They're not! In your examples, the verbs "booked" and "bought" that follow the perfect auxiliary "have" are past-participles, not past tense forms, though for most verbs the forms are identical. "Did" is an auxiliary verb here that requires a plain (infinitival) verb-form like "book" and "buy" in you examples – BillJ Nov 28 '16 at 17:12

They're not in present or past tense. "Do", like most auxiliaries, is followed by the infinitive (the basic form of the verb): "Did you see him? I didn't see him". "Have" is followed by the past participle to form a perfect construction: "Have you seen him? I haven't seen him". The other anomalous auxiliary is "be", which can be followed by either the present participle for a progressive aspect, ("He was painting") or a past participle for a passive ("It was painted").

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