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In the workbook that I'm working with, there is a strange structure of sentence for me and I would like to check with you if it's not wrong or mistake such as typo. The sentence is:

Only after buying the cottage did we discover that it was infested with mice"

The location of the auxiliary verb "did" in this declarative sentence is not clear to me. Normally, when it comes before the pronoun then the sentence is turned into an interrogative sentence.

We can use also the auxiliary verb as an emphasizer but in such cases it should come after the verb (e.g. Only after buying the cottage we did discovered that it was infested with mice"). In addition, I do not understand the omission of the past form (ed) from the word "discover".

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marked as duplicate by Alan Carmack, Brian Tompsett - 汤莱恩, P. E. Dant, James K, Peter Nov 6 '16 at 10:23

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  • It's an emphatic 'we discovered' – SovereignSun Nov 5 '16 at 16:21
  • There are many examples on the web. – SovereignSun Nov 5 '16 at 16:24
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    Your example has subject-auxiliary inversion. This occurs in declarative clauses only when certain types of element (like "only") 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 Nov 5 '16 at 16:29
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    Auxiliary do is always the tensed form in the verb group, and it always takes the infinitive (base) form of the lexical verb as its complement (did discover), never a preterite or participle form--did discovered is ungrammatical. – StoneyB Nov 5 '16 at 16:39