"Sir, do you work on Friday?"
Translation:Señor, ¿usted trabaja el viernes?
- If you are talking about the gentleman, you use el.
- If you are talking to the gentleman, you don't use el.
Here's a hint to help you remember: if you can substitute "the gentleman" for "señor" or "señor [last name]" and the sentence works, you need to use el. For example, if you want to say "Mr. Romero will arrive at two", you could also say "The gentleman will arrive at two", so it's "El señor Romero llegará a las dos".
But if you are saying "Sir, sign here, please", you can't say "The gentleman, sign here, please", so there's no "el"--it's just "Señor, firme aquí, por favor".