We start a sentence with the base form of a verb when we want to tell people what to do. e.g. put up a picture on the wall.  paint the walls blue. We add do not/don’t in front of a verb when we want to tell people not to do something e.g. Don’t paint the cat. We often add please in a sentence to make an instruction more polite. e.g. Please fix the shelf for me.  Don’t touch the wet paint, please.