Expressing oppinions is one example of culture difference between east and west. Westerner express their oppinions freely. They feel that it is natural for people that have different ideas and don't mind hearing you that differ from their own. You may express frankly what you think  or feel. Of course, you shouldn't be rude in your manner of speech. Statements such as " You are wrong", "You are mistaken", "You don't know", etc are exactly bad. But it is preferable to soft your  tone. You might say, " I don't think you are right", " I am fraid you are mistaken", or "You don't seem to understand".