Subject + 忽然 + Verb
The adverb 忽然 (hūrán) means "suddenly" or "all of a sudden," indicating that something happens unexpectedly and without warning. It is placed before the verb in a sentence. It is similar to 突然 (tūrán) but tends to be used more in narrative and written contexts.
我们正在聊天,他忽然站起来就走了。
Wǒmen zhèngzài liáotiān, tā hūrán zhàn qǐlái jiù zǒu le.
We were chatting when he suddenly stood up and left.
天空忽然下起了大雨,大家赶紧找地方躲。
Tiānkōng hūrán xià qǐle dà yǔ, dàjiā gǎnjǐn zhǎo dìfang duǒ.
It suddenly started pouring, and everyone rushed to find shelter.
她忽然想起今天有个重要的会议。
Tā hūrán xiǎngqǐ jīntiān yǒu ge zhòngyào de huìyì.
She suddenly remembered that she had an important meeting today.