I hate to say it, but your editorial is going to sound very school newspaperish if you use 1st person.
How do I do to know? It confused me for a bit, but I think you mean “How do I know when to use”. You use infinity when the number is countless and gerund when the verb is noninfinite.
Answer:
what do you want me to do with this story
The answer is B. first, last, and main words in a book title.
Basis of actions or issues is the answer! Next time try crossing out the answers you don't think are the answer and that might help