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 ...

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 ...

A fronted adverbial is when the adverbial word or phrase is moved to the front of the sentence, before the verb. Earlier today, I discovered fronted adverbials. So here, 'earlier today' is a ...