all she does is cry←省略了to?为什么可以省略?

all she does is cry←省略了to?为什么可以省略? 她中等身材.She is of medium build.里面的of省略了可以吗? It is known to all that he is a good student.这里的that可以省略吗? Courage is knowing what not to fear .what后省略了什么?为什么有to life is short,so cry while you are youth省略主语了吗 为什么有时候可以省略介词? which is为什么可以省略?有固定语法吗? He found it difficult to sleep.为什么is省略? 英语从句省略thatall you need is love 从句省略了that?原句是that all you nee All we need is enough money.这里的all后面是不是省略了that? 是非限定定语,省略了which吗?为什么可以省略,什么情况下可以省略.请讲通俗易懂点.举个例句分析最好. be able to可以省略to么?