Answer: If, after and whenever.
Explanation: Although the word after can also be used as a preposition ("We ate dinner after the game") or a conjunctive adverb ("We first played the game. After, we had dinner"), you can also start a subordinating adverb clause with it ("After we played the gamed, we had dinner"). If and whenever are frequent subordinate adverbs. Therefore can only be used as a conjunctive adverb ("We won the game. Therefore, we ate dinner at our favorite place.")
