歯 is one of the most commonly used Japanese kanji, and it also has a lot of special readings in addition to the standard onyomi and kunyomi readings.
ONYOMI 音読み
よわい yowai は ha よわ.い yowa.i よわい.する yowai.suru
KUNYOMI 訓読み
シ shi
Examples
その赤ちゃんは歯が生えはじめている。The baby is cutting his teeth.歯が痛い。I’ve got a toothache.歯が痛いです。I have a toothache.ひどく歯が痛い。I have a bad toothache.私の前で歯をほじるな。Don’t pick your teeth in front of me.私はきのう歯が痛かった。I had a toothache yesterday.冷たいものが歯にしみます。My teeth are sensitive to cold foods.彼はほとんど歯が無かった。He had few teeth.私は歯医者で歯を調べてもらった。I had my teeth examined at the dentist’s.歯をきれいにしておかなければならない。You must keep your teeth clean.