1.表示说话时正在进行的动作

常和now连用，有时和动词如look，listen等连用，表示“现在”这一概念：

Listen! Birds are singing.

听!鸟在歌唱。

Look! he train is just getting into the station.

看!火车进站啦。

He is reading now.

他在看书。

The river is flowing fast after last night's rain.

昨夜下雨后，河水流速很快。

The house is being painted.

房子正在上油漆。