标志：be + 动词的现在分词

1.  表示说话时正在进行的动作：

She is writing a letter upstairs.

Who are you waiting for?

It is raining hard.

2.  表示现阶段一直在进行的动作（说话时动作未必正在进行）：

I hear Mr. Green is writing another novel.

3.  表示反复出现或习惯性的动作，往往包含说话者赞扬、责备、厌恶等情绪，通常与always, constantly, continually, forever等频度副词连用。如：

John is forever asking silly questions like a stupid.

He is always thinking of others first.

4. 表示将来

1)  表示按计划、安排将要发生的动作，仅适用于部分趋向动词（如go, come, leave, start, arrive等）。如：

Uncle Wang is coming.

They're leaving for Beijing.

2)  在时间和条件状语从句中，现在进行时表示将来某时正在发生的事情。如：

Please drop in when you are passing my way.

If he is still sleeping, don’t wake him up.