Please see the section "Verbs" of the Tips and Notes for the very first unit: https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes
This is covered in the Tips & Notes. If you have not been reading the Tips & Notes, I would like to ask that you review those before asking your question here so we don’t have to continuously repeat the information that we have explained there. If you have read them and still have a question, please be more detailed in your question so we can figure out where the confusion is.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to use a web browser, either from your mobile device or from a computer. When you click on a Skill, it will expand to reveal a Start button and Tips button. If you click on the Tips button it will give you a detailed explanation of the grammar that is introduced in that Skill. If you still have questions after reading the Tips & Notes for the previous and current Skill, then please ask your question again, explaining what you didn’t understand or what seems contradictory to you.