"This is my teacher."
Translation:Це мій учитель.
I do not quite understand what the problem is. If you have to translate "teacher" into Ukrainian and you do not know the word then you can always hover with the mouse pointer over the word to get a tip.
On the mobile app, this was one of those exercises where you pick the words from a list to construct the sentence. That type of exercise doesn't have the ability to see translation tips.
Both, there are some situations where you can switch "в" and "y".
Both variants are accepted, but the best one is "учитель," because the previous word ends with a consonant (й).