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口内炎が

a canker sore

→ 口内炎
phrase Rare #33,131
a canker sore 口内炎 means “canker sore” or “mouth ulcer,” and が marks it as the subject. This phrase is usually followed by a verb such as できる, meaning “to develop” or “to get.”
JA: 口内炎ができて、食べるのがつらい。
EN: I have a canker sore, so eating is difficult.

口内炎ができているのです。

I have a canker sore.

口内炎が痛くて、あまり食べられないんだ。

My canker sore hurts, so I can't eat much.

また口内炎ができちゃった。

I got another canker sore.

私は唇の中を噛んで口内炎ができた。

I bit the inside of my lip and got a canker sore.

口内炎ができて食事する時、痛くて痛くて。

I have a mouth ulcer, and it hurts terribly whenever I eat.