Remember that the mistake I mention above is often made by beginners. While it is true what you said, it is easier to just tell them: always use two verbs in a question or negative sentence. If you don't have two, just borrow "do" and apply the appropriate time/person to that verb and use the infinitive form of the second verb. When learning a language it is important to have easy-to-understand and simple rules. When you try to write or speak in a foreign language, there are a lot of things to consider and you are going to forget a few. The best thing you can do is practice a lot until it starts to sound natural. If you always try to apply all rules you've learned, you are going to miss some, no matter how smart you are. It's also important to have someone who can correct you and tell you the appropriate rules.