An adverbial is an adverb, adverbial phrase or adverbial clause which gives us additional information about e.g. the time, place, or manner of the action which is described in the rest of the ...

A relative clause can be introduced by either a relative pronoun or a relative adverb. A relative clause can be restrictive or nonrestrictive (essential or nonessential) Restrictive relative clauses ...

In this case, ‘Not until I saw what happened’ is the adverbial clause. The inversion takes place after this, in the main clause. This happens with 'Not +time' and 'Only + time'. 'Not since ...