动词ing形式的用法
a.表示现在( 指说话人说话时) 正在发生的事情.
We are waiting for you.
b.习惯进行：表示长期的或重复性的动作,说话时动作未必正在进行.
Mr.Green is writing another novel.
(说话时并未在写,只处于写作的状态.) She is learning piano under Mr.Smith.
c.表示渐变的动词有：get,grow,become,turn,run,go,begin等.
The leaves are turning red.
It's getting warmer and warmer.
d.与always,constantly,forever 等词连用,表示反复发生的动作或持续存在的状态,往往带有说话人的主观色彩.
You are always changing your mind.