I can see that the usual translation "only" would work for Irish and UK English speakers, but I'm starting to suspect that there is more divergent evolution between US and UK/Irish English than is usually discussed.
Realizing also that my own language is divergent from American standard I would say "Merely" is a better fit for me, but I memorized the usual translation.
I think that rote memorization is actually one of the strengths of this course. If you think about it, babies learn their native language by brute force memorization.
I translated the sentence as "they are not but bears" and was marked wrong, which is wrong, and reported it. "They are not but bears" or "they are nought but bears" are correct translations. Though I am fond of scilling's literal translation. "Is nought in them but bears!" 100% ursine essence : )
In the last 10 years, how often have you used the construction "they are not but ..." and how often have you used "they are only ..." in real life?
The purpose of an exercise like this isn't just to teach you how to read an Irish phrase, in which case a crude "literal" translation will do, but to teach you how to translate your daily idiomatic English into Irish, in which case that crude literal translation is less than useless, because it doesn't help when you want to translate "they are only ..." into Irish.
There are instances when a "literal" translation is appropriate, and there are times when a "literal" translation will impede your learning progress. This exercise is one of the latter.
That is slightly complicated. The Irish for "He is the only bear (in the zoo)" is Is é an t-aon béar (sa zú) - or more literally, "that is the one bear in the zoo".
You can't use an t-aon with the plural béir, so you say níl aon bhéar eile sa zú - "there isn't any other bear in the zoo". You can add ach iad/"but them" for emphasis.