"The children eat the apple."
Translation:Những đứa trẻ ăn táo.
This is why I disagree with the convention in these lessons that the unqualified Vietnamese nouns are translated as "the [noun]". When Vietnamese people say "Những đứa trẻ ăn táo" they clearly mean that the children are eating apple, not that they are necessarily eating that same single apple. The English translation offered here misunderstands the intentional ambiguity of the Vietnamese and constrains the meaning more than the English requires with its uncareful translation.
In English "The children eat the apple" normally means plural children eat a singular apple, that is, they share one apple. Is this what the Vietnamese means? (Otherwise, "the apple" specifies "apple" as a particular selection among a range of possibilities and says nothing in particular about the number of apples at all.)
Unlike in English where "apple" means the fruit, táo really means the apple as the species. Trái/quả is the classifier for round objects - you need to use trái táo for the fruits of the apple when counting. And you would you use the classifier cây, you're referring to a plant/stick/rod/tree of something instead, i.e. cây táo - apple tree. Cái is a nonspecific classifier, used for those that do not have any specific classifier. But especially since táo can be used with many classifiers it is not really a good fit.
Lastly it can be used with other classifiers as a focus marker, like, cái trái táo này = this particular apple fruit. You can even drop the word for apple altogether: cái trái này - this very fruit.
"quả" or "trái" are clasiffiers for fruits. Sometimes you can omit it, but you cannot omit it when you talk about the quantity.
"one" and "a"/ "an" are used to indicate the quantity and it means "một", and "the" is used to indicate the specific thing you want to talk about, and "the" is used for both singular and plural.
As this sentence above: "The children eat the apple". (Những đứa trẻ ăn táo), you do not know exactly how many apples the children eat, so you cannot use "một" (a/an/one) here. But in this sentence, it has the article "the" to indicate a specific fruit that children eat, it is "apple" (the apple).