10. d. Jason gets stage fright so he decided not to audition for the play.
When the coordinating conjunction <em>so</em> is linking two independent clauses (clauses that can stand alone and have full meaning), a comma is necessary. This is also true for other coordinating conjunctions like <em>and, but, for, or, nor, yet, etc.</em>
Independent clause 1: <em>Jason gets stage fright</em> and 2: <em>He decided not to audition for the play.</em>
11. a. You must return your first rental, before they will send you another movie.
When the preposition <em>before</em> is placed at the middle of a sentence and it is not an appositive phrase, it should not be preceded by a comma. In this sentence, there should not be a comma. Now, if the preposition was placed at the beginning of the sentence, it requires a comma, for example: Before they send you another movie, you must return your first rental.
12. b. I know how much they usually cost?
This is a statement and not a question (it does not begin with an auxiliary verb such as do, be, have), thus the question mark there is misplaced here.
13. a. Doing my homework, making my bed, and practicing guitar is on my to-do list for the day.
This sentence is correct as it is using the commas to separate more than two items or activities in a sentence, which is one of the many functions of a comma.
