当她不小心踩到钉子时，忍不住骂了几句脏话。

She couldn't help but ___swear when she accidentally stepped on a nail.

他举手发誓会保守这个秘密。

He raised his hand and ___swore to keep the secret.

他发誓他从没见过那个陌生人。

He ___swears he's never seen that stranger before.