"You do this."
i can interpret this in two ways:-
- i am asking someone to do something it is command.
- i am talking about his usual habit like "he always does this"
then how to indicate in which sense i am talking in?
"You, close the door" is a command, and the comma represents what would be a slight but noticeable pause in speech.
"You close the door" would be spoken without a noticeable pause between "you" and "close." Such a sentence may be either a description of habitual or regular action or the historical present.
The example is highly artificial because it is a very brief sentence given without context. In practice, either the sentence itself or its context would usually give additional clues to the intended meaning.