Expressing opinions is one example of culture difference between east and west. Westerner express their opinions 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 afraid you are mistaken", or "You don't seem to understand".