Expression "unless he be a lunatic"

  1. No one, unless he be a lunatic, would do that.
  2. No one, unless he should be a lunatic, would do that.
  3. No one, unless he is a lunatic, would do that.
  4. No one, unless he were a lunatic, would do that.
    Which of the above sentences is NOT acceptable?

What about “No one except a lunatic, would do that” ?

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Of your 4 sentences, only #3 does not present a hypothetical lunatic.
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Did you mean that only #3 is not acceptable?

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#3 is quite common, but if this is a test question, then it is the unacceptable one. Strict grammar asks for the hypothetical here because of the ‘would do’ conditional.
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Hi MM,
could you please be a bit more detail about your explaination?

1/ Can ‘unless’ be used with the same structures as IF (3 types)?
2/ Except #4, I find the other three sentences a bit strange… could you explain the difference among them?
3/ What’s the difference between the version ‘No one, unless he were a lunatic, would do that’ and the version ‘No one, unless he is a lunatic, do that’?

Many thanks in advance,
Nessie.