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I have always thought until just before that a verb should come before the subject only in questions, for example:

Is this idea correct?

But I was just reading a book ("Operating System Concepts" by A. Silberschatz and others, 9th ed., chapter 6.3.6), which says:

Only when queue 0 is empty will it execute processes in queue 1.

Why does "will" come before "it" in the sentence above?

Could I have said:

Only when queue 0 is empty it will execute processes in queue 1.

Is there any difference between the two?

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When an adverb or adverbial phrase is inverted and placed at the start of a sentence, you need to change the word order as if you would write an interrogative sentence as in your question.

In your example sentence, only is being emphasized and placed at the beginning making the sentence inverted. The non-inverted (normal) sentence would be:

It will execute processes in queue 1 only when...

Now, you would ask why you need the inversion? The link explains well how the inversion works and what kind of adverb or adverbial phrase requires the inversion. For example:

He not only asks a lot of questions, but also he answers a lot.

could be inverted to:

Not only does he ask a lot of questions, but he also answers a lot.

The purpose of this inversion is to emphasize the adverb or adverbial phrase that is used. There's no big difference in meaning between the two.

You can also visit the Wikipedia link about Inversion (linguistics).

  • I'd like to add that, if we start a sentence with Not only ..., the inversion is required. I don't think you missed this point, but it's easy for the reader to miss this important point, IMHO. – Damkerng T. Apr 17 '16 at 13:47
  • Constructions like these are more common in writing than in speech, though they do occur in speech. – Colin Fine Apr 17 '16 at 21:41

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