Also, a quick question: shouldn't it be A zöld és a piros almák is esnek?
Why "apples" is in singular?
Why the accusative? "Alma" is the subject. As for the plural, I guess you are wondering the same as I am, only from the opposite point of view, that is, translating from English to Hungarian.
Oh, I accidentialy wrote almát instead of almák, I was wondering about plural, yes.
The Hungarian sentence is using the singular because you have two very distinct apples - exactly one red, and exactly one green. You can express that by saying "a zöld alma és a piros alma", but that sounds a little redundant, so you usually just leave out one of the instances of alma: either "a zöld és a piros alma" or "a zöld alma és a piros". The fact that you use the definite article a twice here tells you that it's two different apples.
But the English translation suggests "apples", which means any amount of red and green apples to me. (But it accepts apple too)
"The red and green apple" should be the translation?
I also realized now that esik is singular, for two apples? How come? Should it be esnek?
I know that you can do this in German, implying that two things or persons are almost unseparable and hence become one. But hardly with apples.
Or is it one apple is falling, and the other one is falling too and when we combine both events inside that sentence, it is still shown as singular?
You have to look at how the Hungarian sentence handles its articles. Here it says "a zöld és a piros alma". You have the article a twice, so the green apple and the red apple are different ones. If it were the same apple, you'd say "a zöld és piros alma" - "the red and green apple".
Why isn't it more than two apples? Technically it could be, but that would make the Hungarian sentence sound like a general statement - some truth about all red and green apples - and translate to "green and red apples" instead, without "the". But if it actually were multiple green and red apples falling here, I'd also use plural in Hungarian: A zöld és a piros almák esnek.
I'm not exactly sure how English handles such a distributed plural - they generally avoid that. In German you can say "der grüne und der rote Apfel", using a grammatical singular to describe two apples, exactly like in the Hungarian sentence. In English it could either be "the red and the green apple", using the same method as in Hungarian, or "the red and the green apples", which makes the plurality clear but is ambiguous about the exact number.
You could use esnek here, since you have two subjects - "a zöld" and "a piros alma" - but usually when something like that happens, when you have two (or more) singular 3rd-person subjects, you still tend to use singular conjugation: János és a lány buszra vár.
I don't think the plural is correct here. In Hungarian it is singular, and there are no numerals, indeed there are definite articles, so there should be one green and one red apple.
Duo wants 'the green and the red apple fall too'. I think it should be either 'the green and the red apple falls too' or 'the green and the red apples fall too' otherwise we end up with quite a marked Hungarian accent.