"We cooked lunch."
Translation:Εμείς μαγειρέψαμε μεσημεριανό.
Me three :-) Μαγειρέψαμε το μεσημεριανό was rejected as wrong.
Well I would have expected that to be implied, as you're cooking today's lunch, so it is "the" lunch, not "a" lunch, i.e. any indeterminate lunch, and usually Greek seems to use the definite article where it is often left out in English (unlike the indefinite article, where it seems to be the other way round).
For example, I translated an exercise today in the form of "scientists do something or other" (I forget the detail), and I translated with no article, but was corrected to "οι επιστήμονας..."
Yes, the definite article is needed in many cases, but in this case using it would give emphasis that it is the Lunch that we cooked and not something else, and in that case in English we would say "We cooked the lunch", no? I think that the only difference between the uses of the definite article in Greek and in English is the fact that in Greek, it is used for general meanings. Οι επιστήμονες σε όλο τον κόσμο=Scientists all over the world.