我肯定是他偷了我的手机。

Wǒ kěndìng shì tā tōu le wǒde shǒujī.

I'm certain he stole my phone.

他肯定不会让我失望的。

Tā kěndìng bú huì ràng wǒ shīwàng de.

He definitely won't disappoint me.

我肯定我没记错。

Wǒ kěndìng wǒ méi jìcuò.

I'm positive I didn't remember wrong.