"We use the girl."
Translation:Úsáidimid an cailín.
Correct solutions: • Úsáideann muid an cailín. • Úsáidimid an cailín.
I see both forms are accepted now - great.
A noun gets eclipsis if preceded by both a preposition and a definite article. An cailín; ag an gcailín.
Was this what you were thinking about?
Why not "an chailín"? Aren't feminine nouns lenited after "an" nominative?
To elaborate on this, it's because of the diminutive ending -ín. When it is added to a word, the word becomes masculine (it's a neat trick you can use if you're having trouble!). Also, note that cailín literally means "little maid" (caille + -ín)