So just because you teach English may not mean that you understand it, especially if you are a native speaker and don’t have special training in teaching English. Here are some great sites to look up all sorts of rules and guidelines for the English language:
About.com: ESL: includes charts, quizzes and other related resources which review and introduce grammar.
The Basic Elements of English: Guide to better understanding parts of speech, sentences, punctuation and word use.
Grammar Bytes: Contains a wide variety of terms, exercises, handouts, presentations, tips and rules addressing English grammar.
Guide to Grammar and Writing: Breaks down topics by word/sentences, paragraph, and essay levels
Oxford Dictionaries Online: Good for teacher and student alike, this includes a dictionary, a guide for better writing, and puzzles and games.
Purdue OWL Writing Lab: Containing a HUGE variety of resources for writing teachers, this site is packed with information for teachers, tutors, and students.
WordReference: a multilingual (15 languages!), interactive, online dictionary. The best thing about this site is the forums where you can ask specific questions about word use, etc.