"Thank you, teacher."
Translation:Tapadh leibh a thidseir.
because your answer has a spelling error in it.
You are correct that tidsear is the word for a teacher in Gaelic. However, tidsear is a MASCULINE noun. In the vocative case, three things happen.
1) the noun gets an a added to the start (exceptions being words which start with a vowel) 2) the noun lenites (exception being nouns starting with letters which can't lenite) 3) masculine nouns slendarise their endings.
So the correct answer is "A tHidseIr" (corrections in capital letters as it won't let me do bold text for some reason).
The way to get
**bold text** bold text is to use double asterisks, so
Note that I have also demonstrated how you can put something in
_italics_ italics by using underscores. Some people may have seen either symbol used for either typeface, but this causes confusion for both people and Duolingo - the method here is the accepted one.
There's lots more formatting here. D