It's not the core of the matter, but would you be referring to one apple, apples plural, a particular apple, or merely some apple (as in French 'des pomme')?
(Incidentally, thank you so much for creating this course! I'm just nitpicking.) :)
EDIT: I think I might be able to answer my own question now. One of the moderators seemed to confirm in another comment section that the object (elma) is automatically plural if there is no 'bir' before it. Apparently we get an explanation in the Food skill. (Sorry - I can't figure out how to link to comments.)
About plurals in Turkish: When you add a number in front, the root does not take a plural suffix.
So, unlike in English, where you would say "one apple, two apples, three apples, etc", in Turkish, you would say "one apple, two apple, three apple, etc" (notice the lack of the plural-making -s at the end.) So you would say "bir elma, iki elma, üç elma, etc.
HOWEVER, you CAN say "apples" by itself (without a number specifier). As in, "the apples on the table" (masadaki elmalar), or "those apples" (o elmalar).
In terms of the suffixes for making plural forms, there are only two possibilities: you can either use -ler or -lar.
What determines when it's -ler vs -lar? A little rule called "vowel harmony", which you'll pick up later. Basically, in order to avoid the vocal strain caused by constant ups and downs due to switching back and forth between different types of vowels, Turkish has grouped its eight vowels into two groups of four: Aa, Iı, Oo and Uu constitute the "hard vowels". Ee, İi, Öö, and Üü constitute the "soft vowels". In essence, vowel harmony dictates that hard vowels should only be used together and soft vowels should only be used together, and that these two vowel types should not mix together. So a word like "kimono" does not fit vowel harmony rules, because it mixes a hard vowel (o) with a soft vowel (i).
Vowel harmony extends into how word roots interact with their suffixes. In this context, vowel harmony dictates that the vowels found in a suffix ought to be in the same vowel family (hard vs soft) as the last vowel of the root. So, for example, "masalar" (tables) is composed of the root "masa" (table) and the plural suffix -lar. The "a"s correspond to each other (i.e. both hard vowels), and the word thus fits the vowel harmony rules. Similarly, the word "eller" (hands) is composed of the root "el" (hand) and the plural suffix -ler. The "e"s correspond to each other (i.e. both soft vowels), and the word thus fits the vowel harmony rules.
So now that you know the basic rules, what would you append to the roots "tür" (kind/type), "kalem" * (pencil), "kitap" * (book), "yol" (road), "deniz" (sea), and "göl" (lake) to make them plural? You would say türler, kalemler, kitaplar, yollar, denizler, and göller, respectively.
*You'll notice that the roots "kalem" and "kitap" themselves do not fit into vowel harmony rules. This is because they are foreign language imports, and vowel harmony does not apply to the stem of loanwords. It does, however, apply to what suffixes may be appended to those stems, as we saw with kalemler and kitaplar.
I don't think you can automatically assume something is in plural just because there is no 'bir' before it. In the example above "su ve elma" makes me think it's the answer to a question like "what are you having? oh, just water and apple" I may be misunderstanding your question, so feel free to clarify. :)
The problem is, I think, that in English (nor in Dutch or German) you don't say "water and apple". You can say "water" just by itself (it's an uncount noun) but you don't simply say "I'm having apple". You have to use an article "I'm having an apple", "Ich habe ein apfel", "Ik eet een appel", "Je mange une pomme", etc.
HOWEVER, in Irish Gaelic, you don't have indefinite articles (a/an) at all, only the definite article (the). How does that work in Turkish?
When I wrote the example above, I wrote it in English however with the Turkish phrasing in mind. I hope I didn't confuse you there! :)
So, moving on to your second question. You don't need to have definite articles in Turkish. It's perfectly fine to say "Elma yiyorum - I am eating an apple." which would be odd in English. (I am eating apple just sounds wrong!)