Inversion with Only Expressions (Basic)

Expressions beginning with only — such as only then, only when, and only after — trigger inversion when placed at the start of a sentence for emphasis. The crucial rule is that the inversion happens in the main clause, not in the "only" clause itself. Compare the normal order "I understood the problem only after reading the report" with the inverted form "Only after reading the report did I understand the problem." The auxiliary verb "did" moves before the subject "I" in the main clause, while the "only after" clause keeps its normal word order.

A very common mistake is putting the inversion in the wrong clause. "Only when did she arrive, she realised the truth" is incorrect — the correct form is "Only when she arrived did she realise the truth." Notice that "she arrived" has normal word order, and "did she realise" has inverted order. Only then is slightly different because it stands alone without a following clause: "Only then did we understand." These structures are frequently tested in Cambridge B2 First Use of English transformations.