Let us use a simple sentence to explain this ... “Whoever fixed it is an expert”, “what was left of it”. Adverbial clauses are subordinate clauses that function as adverbs in sentences.

A subordinating conjunction is the word/words used to link two clauses together, a main clause and a subordinate clause. The subordinate clause always depends on another clause for its full meaning.

He was annoyed because the train had stopped. Subordinate conjunctions (and clauses) can be used in different places within a sentence. They can go at the start, middle or end of a sentence.