I've learned it is obligatory to invert subject and verb when 'only when' is placed at the beginning of a sentence. However, I've recently found the sentence below.

"Only when the rule can have some meaningful effect it is to be applied."

Some English teacher in my country said this sentence is grammatically correct. He said inversion shouldn't happen in this case because a pronoun 'it' is used as a subject in the main clause and 'it' refers to 'the rule'. I'm not sure whether or not the sentence is correct, but I'm pretty sure that teacher doesn't know what he is talking about.

I think that sentence should be "Only when the rule can have some meaningful effect is it to be applied."

What do you think? Is it incorrect? If so, why is that?

Many thanks in advance.

  • Related: english.stackexchange.com/questions/124595/…
    – user66974
    Aug 8, 2016 at 8:36
  • 3
    You are right. Subject-auxiliary inversion occurs in declarative clauses only when certain types of element are put in front position. Negatives are one very obvious type of element that trigger subject-auxiliary inversion when fronted: "Only" is not negative, but it is semantically close to a negative, in that "Only John liked it", for example, entails "No one other than John liked it".
    – BillJ
    Aug 8, 2016 at 9:14
  • 1
    It's not "only when" that causes inversion; it's just the word "only". If "only" is omitted, there's no inversion: When the rule can have some meaningful effect it is to be applied.
    – user164312
    Aug 8, 2016 at 10:30
  • @user164312 No, it's all the colligations 'only then', 'only after tea', 'only after finishing the housework' ... 'only when he had gone' ... 'only if pigs could fly' ... 'only in this way' ... 'only by trying' ... // and the adverbs 'seldom' // 'rarely' // and negators like 'never' ... that trigger inversion. 'Only' cannot be used alone here, unlike 'seldom' / 'never' / 'nowhere' .... Dec 29, 2021 at 13:43

1 Answer 1


Yes, you are correct. As is the case with other negative adverbials, inversion is required after 'only when ...'.

This grammaring article gives the rule and examples:

When only after, only if, only in this way etc. are placed at the beginning of the sentence for rhetorical effect, the subject and auxiliary are inverted:

Only after lunch can you play. (You can only play after lunch.)

Only after finishing your homework can you play. (You can only play after you finish your homework.)

Only after you have finished your homework can you play. (You can only play after you have finished your homework.)

Only by guessing can you solve this puzzle. (You can only solve this puzzle by guessing.)

Only if everybody agreed would I accept this position. (I would only accept this position if everybody agreed.)

Only in this way does this machine work. (This machine only works in this way.)

Only then did they discover his secret. (They only discovered his secret then.)

Only when he needed some help did he call me. (He only called me when he needed some help.)

Only when I filled my glass did I notice that it was broken. (I only noticed that my glass was broken when I filled it.)

This To Learn English article gives a wider range of negative adverbials.

  • 1
    There is an small apparent exception, where 'only' is used as a conjunction, equivalent to 'but' or 'except', as in CED only conjunction in British English. UK /ˈəʊn.li/ US /ˈoʊn.li/ C2 used to show what is the single or main reason why something mentioned in the first part of the sentence cannot be performed or is not completely true. The definition suggests that this use can only happen as a qualifier to what is said in the same sentence. That is why I have not entered it as an answer. But it could exist as a sentence. "You can come over if you like. Only I might be out.
    – Tuffy
    Dec 28, 2021 at 19:22
  • Yes, colloquial 'Only' = 'But you have to bear in mind that' etc. The mitigator / corrective pragmatic marker (outside the matrix sentence) usage. But the question is about and the answer addresses 'only after, only if, only in this way etc ... placed at the beginning of the sentence'. Dec 28, 2021 at 19:31

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