Answer:
1. While he was taking a shower, Jimi Hendrix had a brilliant idea.
2. After she tried on twelve shirts, Ginny Weasley decided to buy only two.
5. Since we don't have enough people to play charades, Jar Jar Binks suggested we watch a movie instead.
7. If Lucius Malfoy doesn't get here soon, we will have to leave for the zoo without him.
Explanation:
Subordinating conjunctions are used to link a dependent clause to an independent one. Dependent clauses don't convey full meaning and they can't stand on its own in a sentence so they need to be connected with independent clause. General rule is that if a dependent clause precedes an independent in a sentence, it needs to be separated from it with a coma.
That means that sentences 1), 2), 5) and 7) require commas.
