You're and your. Their. There. They're. It's and its. These homophones can sometimes cause huge problems for students! Even if you know the difference between them, sometimes your eye just skims right over it in your writing because you know what you expect to be there - and, because it's spelled correctly, your spell check doesn't pick it up. Therefore, when you are editing your work, it is important to review very carefully what words you're using and whether you're using them accurately in your work.
Read about Homophones in your English Language Arts Handbook for Secondary Students, pp. 11-16. Use your editing strategies to try and identify these problematic words in your writing and ensure that you're using them properly!
Other commonly confused words include affect and effect: affect is to cause a change and effect is
an outcome or result. How about accept and except or buy, bye, and by?