"Do you play tennis?"
Translation:Am bi thu a' cluiche teanas?
Belatedly, I've just looked at my dictionary. Teanas is the nominative, teanais the genitive. Having not reached that part of the course, I have yet to be enlightened about why a genitive might be correct in this context. Both are nouns, though (and I did know that slenderizing of the final vowel is typical of the genitive). No doubt all will be made clear at the appropriate point.