1. Explain it simply
忽然 (hūrán) means 'suddenly' or 'all of a sudden'. It marks an abrupt, unexpected change.
Word order: Subject + 忽然 + verb phrase, as in 他忽然哭了 (he suddenly cried).
You can also front it with a comma to open the sentence: 忽然，天气变了 (suddenly the weather changed).
Keep 忽然 before the verb - do not push it to the end or wedge it inside a verb.
2. Show examples
他忽然哭了。
Tā hūrán kū le.
He suddenly started crying.
忽然 sits before the verb 哭
忽然，天气变了。
Hūrán, tiānqì biàn le.
Suddenly, the weather changed.
忽然 can open the sentence with a comma
车忽然停下了。
Chē hūrán tíngxià le.
The car suddenly stopped.
an abrupt, unexpected event
3. Share a tip
Put 忽然 before the verb - say 他忽然哭了, not 他哭了忽然.