***** NOT A TEACHER *****
The OP has already received excellent answers. I only wanted to expand a bit on the topic, for I am sure that many learners find this matter rather confusing.
I have made up two sentences:
1. Everybody stands up when the Queen enters the room.
2. Everybody stand up when I enter!''
As you, dear learners, can see, #1 uses "s" (the indicative) because it is a fact: Everyone DOES stand up whenever Her Majesty comes into a room.
In #2, however, that is NOT a fact. It means something like: "I demand that everyone stand up when I enter the room." It is NOT a fact. It is only my demand or wish. So we do NOT use the "s." (We use the so-called subjunctive form.)
*****
I have found an explanation from a world-famous scholar. It has really helped me. I am delighted to share it.
Look at his example:
Oh, please, someone go in and tell her."
That scholar says this: "Any imperative [command] is virtually [actually] in the second person ["you"], even if [it is] seemingly addressed [said] to a 'third person' [such as "someone"]."
In other words, the scholar says (that) that sentence actually means something like: "Oh, please, one of you present, go in and tell her."
-- Otto Jespersen, Essentials of English Grammar (1933), page 148.