"The boy puts the butter back."
Translation:Cuireann an buachaill an t-im ar ais.
When you submit an incorrect answer, Duolingo compares the answer that you submitted to all of the "accepted alternative" answers, and suggests the one that is "closest" to the answer that you submitted. But this analysis is not a semantic analysis, it's simply an algorithm that compares the letters used in the various sentences, and comes up with a "distance" between the various sentences.
At some point, someone suggested an alternative answer that included gasúr instead of buachaill (gasúr means "boy" or "youngster"), and your wrong answer was closer to that sentence than to the "standard" answer. Duolingo wasn't really "telling you" to use gasúr, it was giving you a complete sentence that included gasúr from the list of acceptable alternative answers that it will accept as an answer for this exercise (Duolingo doesn't understand either of the sentences, it just knows what's on the list of acceptable answers).
This is a quote from a paper written by a Duolingo Staff member:
If Duolingo cannot parse the precise nature of the mistake — e.g., because of a gross typographical error — it provides a “diff” of the student’s response with the closest acceptable answer in the corpus (using Levenshtein distance).