When do you use ser vs estar?
Ser is used for things of permanence, such as name, personality trait, nationality, etc. Estar is used for temporary things, such as location, mood, weather, etc.
Why are writers told not to split infinitives?
Splitting infinitives breaks the flow of words. It's not a rule set in stone, but in general, if you can avoid splitting an infinitive, you are advised to do so. For example, writing "Then I went to the store" is better than saying "I then went to the store."
How do you turn a fraction into a decimal, and that into a percentage?
You must divide the numerator by the denominator to get a decimal. To get a percentage from that decimal, multiply it by 100.