"Apply the rules of grammar" she says! About English!!! As a very fluent native English speaker, I find this funny. English has multiple sets of rules of grammar, all pretty loose, which bleed into each other unpredictably and are often not so much "rules" as "guidelines", which English speakers bend until the Oxford English Dictionary shrugs and says "New rule, they're saying it that way now." IMO English grammar (and spelling) is best internalized by reading a ton of it until you grok the feel. Unfortunately if you're taking a course that tests you on whether you can answer questions about prepositions, you'll do badly, but you'll actually use the language better than someone who answered the questions.