忽然

忽然

Suddenly.

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

他正在看书，忽然停了下来。

He was reading a book and suddenly stopped.

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

天忽然下起了大雨。

It suddenly started raining heavily.

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

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.