He couldn't be a doctor, he is not wearing a white coat.

I would like to know in this sentence, could we use can't instead of couldn't? and what is the difference?

  • 4
    I would find 'can't' more natural in this context. Apr 9 at 18:48
  • @BOLA123 which book? Where did you find this sentence? What is the context? Please provide a link. From this single quote, at first sight, it would seem to be an error. Normally it would be something like "He couldn't have been a doctor, he was not wearing a coat" or "He can't be a doctor, he is not wearing a white coat".
    – Billy Kerr
    Apr 9 at 19:01
  • @BillyKerr The name of the book is (Longman Advanced Learner's Grammar: A Self-study Reference & Practice Book with Answers Diane Hall and Mark Foley page 176)
    – BOLA123
    Apr 9 at 19:03
  • Couldn't is the past tense of can't. So your cited example is unlikely, because he is not wearing a white coat is present tense, which doesn't really work in the context of what he was (or might have been) in the past. Mixing the tenses in the "opposite direction" makes sense, though: He can't be a doctor, he wasn't wearing a white coat is fine (but obviously the time-frames are differet). Apr 9 at 19:03
  • 1
    ...the very next example in the source is "I couldn't pick up a spider, they terrify me". Nothing wrong with that, either. Arguably using the conditional in these contexts is more "assertive" in denying the possibility of the stated condition, in that you're effectively saying that not only can't it be true in the current context - the conditional implies it couldn't be true under any circumstances. Apr 9 at 19:18

2 Answers 2


'Couldn't' (or could not) can be used about the past or the possible future with conditions attached.


  • I went to the library but couldn't find a book I liked (past)
  • I could never watch a film with violence in it (possible future with condition)

Your example sounds wrong as it doesn't seem like you're talking about the past, because he is wearing a white coat. It doesn't sound like you're talking about a possible future either, because "not wearing a white coat" wouldn't stop someone from becoming a doctor in the future.

It sounds to me like you're commenting on a doctor you are looking at, so it should be:

He can't be a doctor, he is not wearing a white coat.

However, if you were speaking about a doctor that you saw in the past (even the very recent past, like you just met him and he has walked out of sight) you might say:

He couldn't have been a doctor, he wasn't wearing a white coat.


He couldn't be a doctor. Those are crime syndicate tats.

Paraphrase: I don't suppose he's a doctor. Would a doctor have crime syndicate tattoos?

He can't be a doctor.

Paraphrase: No way he's a doctor.

can't is definite and certain; couldn't can be tinged with uncertainty.

But with the proper intonation, can't can express incredulity not certainty:

You're pulling my leg! Him, a doctor? He can't be a doctor. He's only 17.

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