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Hello, I met with a sentence:

Under no circumstances should you lend him any money.

Would you please tell me:

1. what grammar is applied here to place "should" before "you" ?

2. why it is "circumstances", not "circumstance" ?

Thank you in advance!
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KooyeenBut if I don't swap them after "only"... it sounds "less wrong" to me
If only is taken to mean but or except, it's OK as is. The intonation would be different as well.

-- There are a lot of things wrong with America.
-- [Only / But / Except] in America you can buy hamburgers this big!
-- You got me there.

Otherwise, in the meaning nowhere else but, the inversion is mandatory, just as for negatives (It's an implied negative, after all.), and it doesn't "sound less wrong" (believe me!) without that switch.

Only in America can you buy hamburgers this big.

CJ
Comments  
Hi,
it happens when you start a sentence with a negative adverb or adverbial phrase, I think. Something negative that is not also the subject, in other words.

Never in my life have I seen such a disaster.
If I'm not mistaken, it also happens with "only" (when it's not part of the subject).
Only in America can you find hamburgers this big.

As far as I know, it is mandatory to use that kind of structure and swap the verb and the subject around, so the following wouldn't sound natural:
Never in my life I have seen such a disaster.
Under no circumstances you should lend him any money.

I am not sure the same is true of the versions with "only" though...
Only in America you can find hamburgers this big. ??? (Do any native speakers also say this?)

As for the plural, I think it makes sense because it's the same kind of plural you would use in sentences like:
You shouldn't lend him any DVDs under any circumstances.

I'm not a native speaker, so let's wait for a native speaker's opinion. I'm not really sure "inversion" is mandatory in every context, dialect, and register.
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ESLBeginnerwhat grammar is applied here to place "should" before "you" ?
Subject-verb inversion occurs after a negative introductory phrase (or after a phrase with only).
ESLBeginnerwhy it is "circumstances", not "circumstance" ?
It's simply idiomatic to use the plural, although it seems to me that I've seen it in the singular occasionally.

CJ
CalifJimSubject-verb inversion occurs after a negative introductory phrase (or after a phrase with only).
Jim, if I don't swap the subject and the verb around after a negative phrase, it sounds wrong to me:
Never in my life I will try this again!
So inversion in such cases is mandatory, isn't it?

But if I don't swap them after "only"... it sounds "less wrong" to me, I don't know why:

Only in America you can buy hamburgers this big.
So, is it really mandatory after "only" as well? I can't trust my istinct, LOL.

Thanks.
 CalifJim's reply was promoted to an answer.
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CalifJim-- There are a lot of things wrong with America.
-- [Only / But / Except] in America you can buy hamburgers this big!
-- You got me there.
Whoa, true! In that case "only" is on its own and it introduces the whole sentence. Interesting.
Anyway, I was interested in the other sense, "nowhere else but", and now I understand. I'll remember to always use the inversion. Thanks!
Wow. Only could mean "but" ! now I know. Thanks a lot!
ESLBeginnerWow. Only could mean "but"
Yes, but only at the beginning of a sentence and in casual style. Emotion: smile

CJ
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Whenever you start a sentence with a negative word or a negative expression you should invert the subject and the verb