"Sa" is not a word in Spanish, so it is not anything.
It is "pon". Example: Pon la silla en la mesa. "Put the chair at the table".
ven
The affirmative ustedes command of pagar is "paguen."
This can be a little confusing. The form you are speaking of is "sé", which is identical to the first person indicative. To tell the difference, you must have context.
An affirmative "tu" command is what is used to tell a person you are familiar with to do something. Examples include: ¡escucha! (listen!), ¡espera! (wait!), and ¡mira! (look!).
tu as 2 frères (affirmative)
Levántese
Levántese
no estudies
(Tu) Entiendes
Despiértate. Enójate.
Positive command: "come", negative command: "no comas".