"Are they legal?"
Translation:An bhfuil siad dleathach?
Trying to understand the difference here.
In the previous exercise, "An rud nua é?" means "Is it a new thing?" As someone pointed out in the comments, "bhfuil" can't be used because it's an identifying sentence. Is there something about this sentence that makes it non-identifying? Would it have to be something along the lines of "Are they legal people?" for it to be considered identifying?
"legal" is an adjective. daoine is a noun.
Ignoring the interrogative for a moment, you would say tá said dleathach - "they are legal", but you use the copula to categorize a noun, so is daoine iad - "they are people". Qualifying daoine doesn't change that - is daoine dleathacha iad.
To turn these into questions, you use the interrogative form of tá (an bhfuil) and of is (an), so an bhfuil said dleathach? and an daoine dleathacha iad?.
Life is not easy: Why is this adjective not in plural form after siad?
"An bhfuil siad dleathacha?"
EDIT: Tips & notes for colors
An adjective that follows a plural noun has its spelling changed to the plural form of that adjective. If the noun ends with a slender consonant, the adjective is also lenited.
EDIT 2: Google finds 50 cases of "siad dleathach", but only one case of "siad dleathacha", so the later is apparently wrong