From the view point of a teacher of English, the verb conjugation for third person singular is one of the most difficult parts of the English language for students aged 13-15 year-old. They often tell me that they don't understand why they attach "s" or "es" to verbs.

Prepositions are really hard. I still have some doubts from time to time.

For me the most difficult part of English grammar is the usage of the articles such as a, and the and prepositions like of, at, in. I get confused many times when and how to use them. I google the phrase when I get stuck if the usage of the articles or prepostions is right.