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The usage of "It was" with "could we" in the following sentence seems intuitively wrong, but I don't know how to properly explain it:
"It was only after they were observed, could we explain certain phenomena."

These seem better:
"Only after they were observed, could we explain certain phenomena."
or,
"It was only after they were observed that we could explain certain phenomena."

And these seem better yet:
"Only once they were observed, could we explain certain phenomena."
or,
"It was only once they were observed that we could explain certain phenomena."

Could someone enlighten me as to the grammatical concepts at work here?

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    Does this answer your question? Inversion after 'Only When'.This covers inversion after 'Only when / after / once etc they were extinct etc'. ////// With 'It was only after X that', 'we/they/she/the Government /// could do Y' is required. Apr 20, 2020 at 14:44
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    Try turning the sentence round - We could only explain certain phenomena after they were observed. There's no place for it was in that version. Apr 20, 2020 at 14:50
  • Thank you @EdwinAshworth. So, the concept I was missing was subject-auxiliary inversion when "only" appears at the beginning (and only at the beginning) of a sentence.
    – TheBamf
    Apr 20, 2020 at 15:22
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    ... or used the same way after a semicolon etc. Seldom, rarely and negations (Not until he has asked me to forgive him ...) also trigger inversion. Apr 20, 2020 at 15:27
  • A minor note on @KateBunting's comment: changing the order of the sentence makes 'they' (more) ambiguous. In the original version 'they' refers to something in the preceding sentence, not the 'certain phenomena'.
    – TheBamf
    Nov 30, 2020 at 11:19

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