Could, would, and should. Your teachers should have taught you about these words. We use "should" to talk about an obligation or requirement. If you want to cancel a class, you should tell the teacher in advance. If you were my student, I would have taught you these words. We use "would" to talk about choices, and it usually follows "if". If you cancelled my class without warning, I would expect to get paid for it. Maybe you could look in a book instead of asking here? We use "could" to talk about possibilities, what can be done, and it's often used in a sentence with "would". I could tell you to look in a book, but I'm going to tell you the answer. If I could answer every question for every student, I would be very happy. If you have a question, you could look in a book but I would be happy if you asked the question here. All the teachers here should be happy to help you.