Usage of formal and informal forms of Korean

In Korean speech, you need to choose formal or informal forms of the language properly depending on the listener's age, social status, or the relationship with them. Generally speaking when the listener is older or in higher status than you, use the formal form. When the person is younger or between friends, use the informal form. People will think you're impolite if you use the minformal form when you should use the formal form. The safest way is to start with the formal form even when the listener is about your age, and gradually switch to the informal form after you become close with each other. For informal forms, change the verb ending of the formal form in most cases. But notice the following words that entirely change their form.