Answer: The subordinate clause in the sentence provided is "after we had waited for twenty minutes".
Explanation: A subordinate clause is a clause that cannot stand alone as a sentence because it does not express a complete idea. There are three main types of subordinate clauses: adjective clauses, adverb clauses and noun clauses. In the sentence provided, the subordinate clause is "after we had waited for twenty minutes", which is an adverb clause because it is functioning as an adverb and it has been introduced by a subordinating conjunction ("after").
