"The girls like books about animals."
Translation:Is maith leis na cailíní leabhair faoi ainmhithe.
Yes, I just got two of the same sentences to translate from Gaeilge to English exactly as you put it there (with the eclipsis), so on the third question, when asked to translate the same sentence from English to Gaeilge, I typed that same sentence... only to be marked wrong and told that there was no eclipsis... even though the previous two did eclipse it.
colour me confused
faoin is a contraction of faoi an - there is no definite article in the English sentence (and it would be na if there was, because ainmhithe is plural
There is no hard rule preventing vowels coming next to one another. What does happen, though, is that when two vowel sounds clash, a consonant is inserted to make the transition between the vowel sounds easier. That's a purely physical basis for the change, not a simple grammatical rule- lena is simply easier to say than le a. Not all vowel sounds clash, though, and faoin ainmhí isn't easier to say that faoi ainmhí - the two words just flow into one another in normal speech.