I found a sentence in my "Preparation and Practice Exam TOEFL" book a sentence states "Not until 1895 did Cornell University begin to offer a degree in ornithology" . anyone can explain what the word "did" means in the sentence? I mean, as we know "did" is a verb and should have a subject, but in that sentence I found that the subject is "Cornell University" and its verb is "begin". so why does "did" in the sentence exist? what for?
Moving some kinds of 'adjunct' (modifying) phrases from their usual position to the front of a sentence triggers subject-auxiliary inversion, just as in questions. Most negative adjuncts, including those with only, do this:
I will go.
→ [inversionWill I] go?
I will go only if I am paid.
→ Only if I am paid [inversionwill I] go.
If there is no auxiliary in the ordinary form of a sentence, "Do support" is required: the appropriate form of do is brought in as a 'dummy' to supply the auxiliary needed for inversion.
Cornell offers a degree in ornithology.
→ [inversion + do-supportDoes Cornell] offer a degree in ornithology?
This, too, occurs with fronted negative adjuncts:
Cornell began to offer a degree in ornithology only in 1895.
→ Only in 1895 [inversion + do-supportdid Cornell] begin to offer a degree in ornithology.