主+系+表
除了be以外，还有那些动词做‘系动词’？
It gets dark. (It is dark.)
You look tired.
I feel happy.
That sounds interesting.
The food smells good.
The tea tastes fine.
He fell ill yesterday.
We must keep fit.
He stands still.
She seems sad.
My parents become old.
The sky appears blue.
Her face turns red.
You will never go hungry.
Your dream will come true.
Your answer proves right.
不定式做表语：
He is to come.
It is to rain.
He seems to have lost weight.(不定式的现在完成时)