You have work/a job to do

Emiliano from Spain, and friend of the podcast, is reading a book in English and wants to know if the sentence “You have a job to do” is correct or if it should be “You have a work to do.”

Both of these sentences are correct and commonly-used in English:
“You have a job to do.”
“You have work to do.” (We would not say, “We have a work to do.”)

The reason we say “a job” and “work” (without the article “a”) is because “job” is a count noun and “work” is a mass noun. A count noun allows you to put an article (“a” or “the”) in front of it or to put a number in front of it: “a job,” “the job,” “one job,” “two jobs,” “three jobs,” etc. Mass nouns, such as “work,” do not take an article (not “a work” or “the work) and we cannot say “one work,” “two works,” etc.

Both sentences — “You have a job to do.” and “You have work to do.” — mean about the same thing, that you have tasks or work that needs to be completed. However, “a job” may sometimes be used to refer to a specific task, often something that you have already talked about or referred to earlier in the conversation or that both people know about. Here are a couple of examples:
A: “Firing (dismissing someone from his/her job) employees isn’t easy.”
B: “No, it isn’t easy, but I have a job to do and I plan to do it.”

Lucy: “Why are you working so late?”
Jeff: “I have a job to do and I’m not leaving until it’s done.”

“You have work to do” can be used to refer to a specific task, but may also be used more generally to say that this person should be working, not doing something else, like having fun.
— “Why is everyone standing around? You all have work to do.”

Lucy: “Today is Friday. Why can’t we leave work early?”
Jeff: “We have work to do. This is no time to think about goofing off (playing or having fun when we should be working).”

Thanks to Emiliano for the question and I hope this is helpful.

~ Lucy