"We eat an apple."
Translation:Nous mangeons une pomme.
In French, to mean "one each", we use the singular, whereas the English use a plural. Only with common sense, we figure out that actually each one of those people are eating one apple each (an apple being too small for several people). If we wanted to really mean that there (unfortunately) is only one apple for several guys, we would say "nous mangeons tous la même pomme", or something like that.
I partly disagree.. Why it shouldn't be 2 persons eating an apple? I don't see the problem. "Nous mangeons une pomme" can mean there is only an apple to share (2 people can share an apple, it's not so weird) or can also mean "we eat an apple each" For "we eat an apple", I don't know because I'm not fluent in English... An English teacher could explain us. If you want really mean each person has an apple, you can precise in French: "nous mangeons chacun une pomme" or "nous mangeons une pomme chacun".
The verb is conjugated with a different form for each pronoun (unlike English where only 3rd person singular takes an -s).
It can happen that two conjugations are similar (like je and il/elle/on below) but nous and vous are always very distinct:
je mange, tu manges, il/elle/on mange, nous mangeons, vous mangez (polite singular and plural), ils/elles mangent.
on mange une pomme = a bit more colloquial, not the kind of thing to write at school when you write a dissertation, and as Sitesurf said, "on mange une pomme" can be a generalization, like the "one" in English. But orally, the "on" is sometimes more used, because it's more simple to say.
Why is "la lune" (the moon) feminine and "le soleil" (the sun) masculine?
Your question has some answers in etymology (Latin, for the most part), but then why did some languages develop with genders and others not?
At the end of the day, you will have to learn each noun with its gender.
A tip: learn them as follows;
- apple = "une-pomme",
- egg = "un-oeuf",
- beer = "une-bière",
- etc...as if every word started with an extra syllable.