Please read the tips and notes before starting a new lesson unit, as these explain basic grammar and sometimes vocabulary presented in that unit.
On the Duolingo website https://www.duolingo.com/ , when you select a unit, don't press Start immediately; instead, click on the little lightbulb icon:
That will take you to the tips and notes for that unit.
If something in the tips and notes is unclear or contradictory, or if there is something you still do not understand after having read the tips and notes, feel free to ask.
Also, if you haven't read about the known issues this course has, please do so: https://forum.duolingo.com/comment/26778885
The big ones are "no sound" (yet) and the fact that hints don't work properly for words that start with an apostrophe such as 'oH.
If you're using an app, my personal recommendation is don't -- use the website instead. But if you do use an app, at least read the tips and notes in a browser first before starting a new unit.
That's not a simple question to answer. This whole long course is designed to teach you that slowly over time. Have you read the Tips & Notes? If you don't know about the Tips & Notes, read this entire discussion thread for instructions how to find them. We're happy to answer questions here, but your question is just too broad and general. Can you be more specific for what you are not understanding?