"The man and the woman eat dinner."
Translation:Mannen och kvinnan äter middag.
I think middagen is the definite way to say "middag". So, generally speaking, dinner is an object here, so they are eating dinner. While if you want to define it, like for example, "the dinner", you would go for middagen. -en in Swedish is the suffix to define masculine names (e.g.: man - mannen). It's just my best guess, it's my second day learning Swedish :)