If the assertion is true, you must back it up with proof or evidence to back up your claim. If the statement is an opinion, you must give an explanation or evidence to persuade your reader. Choose your tone so that it sounds convincing enough to persuade your reader to agree with you. You can do this by using words such as probably, often, always, never, all, some, no, many, few, whole, part. For example, if you are writing about how students behave in class, you would need to show evidence of this behavior being typical. You could say, "Students usually sit quietly in their seats during class," or you could say, "Students rarely get up and walk around during class." The first sentence expresses a fact while the second sentence expresses an opinion. You should also explain why you believe what you are saying. For example, if you are writing an essay explaining why you think President Obama is doing a good job, you would need to provide examples to back up your argument. The first sentence explains why you agree with someone else's opinion; the second sentence explains why you think what you are saying is true.