一、现在此刻正在进行的活动

what program are you watching?
i am watching Friends.

二、目前一段时间内持续的一种暂时情况

i am taking the bus to work this week,because my car is in the garage.

三、用于“变化”类动词（change,come,get,become,grow,deteriorate），表示现阶段逐渐变化的过程。

it's getting dark
mom is getting old
his health is deteriorating.

四、口语说话时用于强烈的感情色彩

what are we waitting for then?let's get started.
i'm not having this conversation with you.