After reading everybody's comments, including PatHargan's and scilling's reply to Pat, I am still not understanding why "I pay (to) the girl" is apparently not a correct translation. The duolingo translation of "don" states that "to the" is a possibility. So...if that is correct, why can't this Irish sentence possibly mean "I pay the girl" (because she's a cashier, for instance)? If not, why not? And if not, what would be the correct Irish translation of "I pay the girl" (complete sentence please) ? Thanks!
I drew a big venn chart from everything I found and I'm not sure it's right. Some seem to do nothing, some just definite eclipse, some just indefinite lenite, many both, a few weirdos, a few it's hard to clasify cos the prep eats the an (like sa = i+an, wtf?). Do duolingo do wall charts? :-)
As already explained:
"I pay for the book" - Iocaim as an leabhar
"I pay for both of us" - Íocaim don bheirt againn
"I pay the girl 20 euro" - Íocaim fiche euro leis an gcailín ("I pay €20 to the girl")
"I pay the driver" - Íocaim an tiománaí
This is an Irish to English exercise, and the Irish is quite clear, but English "for" is ambiguous when used with "pay". the most straightforward meaning of "pay for (person)" is that you are paying that person's share of the bill, and that's what the Irish sentence means.
OK, so it's the English that is ambiguous. But as the translation is from the clear Irish into English, in order to allow us students of Irish and native speakers of English to understand the actual meaning of the Irish sentence, it would be more helpful if the English translation were clearer. A note on the more precise meaning, after the English translation, would be helpful-- as in: "I pay for the girl" (meaning, I pay on her behalf, as in paying for her ticket or for her meal).