Ce sont des garçons - accepted "they are boys". But when I wrote "they are apples" for "ce sont des pommes", it gave me incorrect saying "these are apples". Either is correct in English, so why is "ce sont" acceptable for "they are" for boys, but only "these are" for apples?
You most certainly can use "they" for inanimate objects.
Incidentally, I just put "they are apples" and it was accepted for this exercise (March 19, 2015).
It seems to me it would be a perfectly good response to:
"What on earth are those things on the table?"
"They are apples."
Here's Cambridge's first definition of the word "they"
used as the subject of a verb to refer to people, animals, or things already mentioned or, more generally, to a group of people not clearly described:
I've known the Browns for a long time. They're very pleasant people.
Where are my glasses? They were on the table just now.
They (= people who know) say things will be better in the new year.
You ask an interesting question. Actually "des" doesn't really mean "some". However in some cases we have the option of adding "some" if we like (as long as it doesn't change the meaning of the sentence).
So "Je mange des pommes" means "I am eating apples". If we like we can add "some" and so it becomes "I am eating some apples"
It makes no difference to the meaning of the sentence. In fact "some" is added so often in this type of situation that many learners think that "des" = "some" and many teachers tell learners it means "some" just to simplify the explanation. In this type of sentence Duo always accepts both options (with or without "some").
The sentence in this particular exercise is different.
"Ce sont des pommes" means "these are apples".
OK, so far the same as my previous example but can we add "some" without changing the meaning of the sentence in this case.
No we can't because "These are some apples" has a different meaning. It means "these are very nice apples" or "these are great apples". The meaning isn't specific but it means that there is something particular about these apples.
That is why Duo marked you wrong for adding "some". ;)
How about just accepting that that is not the translation for the French sentence given. This is just a case of French must use an article while English does not. If they wanted you to write "some apples", they would have said quelques pommes.
Sing songs of joy= Chantez les chansons de joie
Notice an article is needed in the French. Not so, the English.
Boys will be boys = les garçons restent des garçons
Here's a different perspective, Philip. "Des" is used here as a plural of "un/une". There is no equivalent in English for such a word, although occasionally one may use the word "some". Do not construe this to mean that "des" means "some". When it is used in this sense, the "some" in English may fit grammatically, but it is almost always omitted. One way to remember it is to ask yourself, what is the plural of "a"? When you say "literal" translation, I presume that you mean you feel the need to take each word in French and find a counterpart in English to represent it. And that is where it breaks down here because in English, there is no answer.
Because "these are some apples" does not mean the same thing as "these are apples". The former implies there is something special about the apples. The French sentence is nothing more than just identifying apples. You have to learn that while French uses articles before nouns, English does not.
L'amour est belle = love is beautiful
Les garçons restent des garçons = boys will be boys
Du vin = wine
J'aime les fruits = I like fruits
Cette is an adjective like "big" and comes immediately before a noun, so cannot stand alone like ce which is a pronoun like "they".
Cette pomme = this/that apple
ce sont des pommes = they/these are apples