"A hard work."
Translation:O muncă grea.
-A- "Muncă" is a work , it means it is not necessary a job with a boss. (1) If you work in the garden : "munceşti în grădină" . "a munci în grădină este o muncă grea!". So "Muncă" is like "work" , it can be something you do , a physical or an intellectual activity and it can be (2) in extension the work you do every day wich means it is you job. -B- "Slujbă" is job and only job. -C- "Serviciu" can be (like in french, "un service" and English, a service) (1) something you do for another person. " Eu fac un serviciu vecinei" : "I do a service for my neighbour, I do a work for her". "you've done me a great service" = " mi-ai făcut un serviciu foarte important" . (2) It can also be your job "Am un serviciu noi" "I have a new job". I go to work (my job)= "Merg la serviciu". You have another word also for work -D- "Lucru" from verb "A lucra" . T=You can say "când mergi la lucru?" "When do you go to work?"
In natural English work is not a noun it is an adjective of which "job" is the corresponding noun.
Similarly in one of your other questions you used the English sentence "I have a very hard work". No, you don't, you have a very hard JOB at which you work very hard.
I hope that makes sense.
To make matters more confusing we might say "at work" (where it is used as a noun, sorry!) and "jobbing", meaning working. Job is a smaller thing than work. Work is a very broad spectrum which may consist of many jobs! At any rate you do not have "a work". You just don't.