Exactly, this is giving trouble to many students that try to build the sentence as in English. We need to understand the meaning and rebuild the sentence using the Vietnamese logic and grammar.
I guess it is hard to understand that in sentences like this "a friend" is not one friend, but friend in general sense, only that English grammar forces us to use the article.
I guess I am lucky because on my mother tongue we have also this sentences without articles and with a similar meaning than in Vietnamese (they are used less often but it helps to understand the mechanism).
The problem we students have, there are often multiple ways one can build sentences like this in English, but we aren't given them and we have to guess. This is frustrating because we usually KNOW the 'right answer' insofar as meaning but DL only accepts a limited range. Thus we are forced to try to spend a lot of effort remembering the only English answer the course accepts. To me this is a case for allowing word-for-word literal translations.
"The enemy of the enemy is a friend" - shouldn't this be at least as correct as the sentence above? There is no một indicating 'an enemy'. Why isn't it:
Kẻ thù của một kẻ thù là một bạn? or ...một người bạn?
This is why, in some cases, using more 'literal' English translations is better or should be at least accepted.
Because in the Vietnamese thinking it's not "a friend". It would be "a friend" if they were talking about one particular guy, but the sentence refer a general statement about friends and enemies.
Literal translation would be "enemy of enemy is friend". That sounds like an Asian guy talking with bad English grammar, and now we know why.
Of course using một or classifier when they are not necessary would sound equally weird to them.
I agree with your observation of the translation of "enemy of enemy is friend" and 'now we know why'. I just wish in such instances we could WRITE 'enemy of enemy is friend" and have it accepted as correct and not have to struggle with/guess what is the only English rendering that the course accepts as correct.
Or, 'enemies of enemies are friends" would fit generic plural rule.