Can't it be 口がいたいんです
「口がいたいんです」- It's because my mouth hurts. (response to a why question, possibly Duolingo's earlier question: 「どうしてあさごはんを食べないんですか？」)
「口がいたいです」- My mouth hurts. (regular sentence or response to "how are you?," does not explain anything)
I put the same since some of the earlier exercises also used the "explanation" particle in this way. I'm not a native speaker but from what I understand it sounds more natural with the "ん".
Absolutely. I reported it.
I put (...いたいんです), and it was accepted.
4-20-19 ; )
How about 口は痛いです？
If you are asked "Do you hurt your mouth?", The only correct answer is.
---+Correct the mistake: Does your mouth hurt?
--->「はい、口 は 痛いです。」
"Do you hurt your mouth" means "Do you like causing your mouth to hurt" in English, and would use 「痛める」 instead of 「痛い」.
Thank you for advice. I sometimes make a mistake in using "hurt".