她忽然哭了。

Tā hūrán kū le.

She suddenly started crying.

忽然下起了大雨，我们决定回家。

hūrán xià qǐ le dàyǔ, wǒmen juédìng huíjiā.

Suddenly, it started to rain heavily, so we decided to go back home.

他忽然间停下了说话，好像在思考什么。

tā hūrán jiān tíngxià le shuōhuà, hǎoxiàng zài sīkǎo shénme.

He suddenly stopped talking, as if contemplating something.