忽然

In a sentence

• 他正在看书，忽然停了下来。

  He was reading a book and suddenly stopped.

  Here, '忽然' is used to describe an abrupt action that interrupts the ongoing activity of reading a book.

• 天忽然下起了大雨。

  It suddenly started raining heavily.

  In this sentence, '忽然' denotes the unexpected commencement of heavy rain.

• 我们谈得很开心，忽然有人敲门。

  We were having an enjoyable conversation when someone suddenly knocked on the door.

  The word '忽然' here conveys the sudden occurrence of knocking amidst an ongoing event.