"Gabh mo leisgeul. Can sin a-rithist."
Translation:Excuse me. Say that again.
While "please" is used as an extremely common marker of politeness in English and its absence might be considered as rude, mas e do thoil e and mas e ur toil e are only rarely used in Gaelic, especially among native speakers. The Gaelic politeness in this sentence comes from the use of gabh mo leisgeul (and the tone of voice in a spoken context). Gaelic also expresses politeness through the use of questions (an can thu sin a-rithist?), different tenses and moods of the verb, as well as negative formulations (e.g. won't you say that again?) which Duolingo hasn't introduced yet - so you will eventually have a relatively wide range of ways of being polite in Gaelic, but adding an equivalent of "please" to a sentence is a rare one.