He believes that the earth is flat.

1-Then he has to be crazy.
2-They he should be crazy.
3-Then he ought to be crazy.
4-Then he must be crazy.

Are all of the sentences 1 to 4 acceptable in response to the first sentence.
Original Post
I would accept only 1 and 4, which represents a logical conclusion, with has to perhaps being slightly stronger than must.

By the way, let's not forget has got to. Smile
We have always been taught that "must" can be used to express "logical necessity" or "logical conclusion", but this is the first time I have known that "have to" or "have got to" can be used to express the same thing! My friend tells me that this is restricted to AmE only, is this true, Richard? I mean is it acceptable to say:
- He has (got) to be rich.
- They have (got) to have stolen it.
Last edited by tonyjab
As I've said before, Tony, I'm no expert on BrE, but I do know they would certainly understand the two example sentences you've given us.

One thing about the second one: It's fine, but I don't think most people would opt to say that simply because it's so wordy. We tend to opt for shorter, more concise phrasing, so saying something containing four syllables (have got to have plus the repetition of one element (have) is not as efficient and easy as simply using two syllables (must have). That's just a hunch, of course.

I honestly do think that those hard-and-fast rules you've come across are changing slowly but surely, Tony.

I'm an ardent fan of a TV show from London called EastEnders. I've been watching that show for something like 15 years! Over the years, I've noticed linguistic changes. One that comes to mind immediately is that they never used to use okay. Where a North American would use okay in a tag question, they would always say yeah or right. For example ...

BrE:
A: I'll meet you at the pub at lunchtime, yeah?
B: Fine. See you then.

AmE:
A: I'll meet you at the pub at lunchtime, okay?
B: Fine. See you then.

But now I hear the British using okay more and more. True, that's only one little vocabulary item, but it's symptomatic of changes that may be happening on one or both sides of "the Pond," as many people refer comically to the Atlantic Ocean.

Anyway, getting back to your two examples, Tony, it's more likely that you'll hear He's got to be rich as an alternative to He must be rich, but He must have/must've stolen it rather than They have got/'ve got to have stolen it.
Thanks Richard! By the way should we use "in" or "on" with the word "expert"?

I am no expert (on/in) BrE.

Is there a difference?
Great question, Tony! Smile

The way I see it, Tony, an expert in something is somebody who can use or participate in doing whatever is mentioned. For example, an expert in English grammar / an expert in needlepoint / an expert in kayaking.

An expert on something is somebody who has a great amount of knowledge about a specific subject. For example, an expert on the Amarna Period in 18th Dynasty Egypt / an expert on DNA forensics / an expert on parliamentary procedure.

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