When you talk about a noun with its quantity, you have to put its corresponding classifier between the quantity indicator (numbers; plural indicators like "những", "các",...) and the noun. E.g:
Một con ong (A bee = One bee), not Một ong
Ba quả cam (Three oranges), not Ba cam
Bốn cuốn sách (Four books), not Bốn sách; etc.
So I hope I can clear up any confusion with this sentence being "A bee" rather than "The bee".
In Vietnamese, you need to include the classifier in every sentence so that it makes grammatical sense. In this sentence for example, "con" is used because it's used for classifying animals.
Here's 2 simple rules to know whether it's "the" or "a".
- If the sentence includes "một" it will be "a", for example in the sentence "Một con ong" it will be "A bee."
- If the sentence does not contain "một" it will be "the", for example in the sentence "Con ong" it will be "The bee."
I hope that helps everyone. =) Have a look at the Tips & notes for some more help with classifiers.