I have a canker sore in my mouth, and it's painful.
ーI have a sore in my mouth, and it hurts.
「口に口内炎ができて痛い」
a sore in one's mouth で「口内炎」
to hurt で「痛む」
ーI have a canker sore in my mouth, and it's painful.
「口に口内炎ができて痛い」
a canker sore で「口内炎」
painful で「痛い」
ご参考まで!