形容词后面可跟动词ing形式，也可跟to do形式。
形容词后面跟ing形式，如：
be worth doing，
be busy doing.
This book is well worth reading.
They are busy sweeping the floor.
形容词后面跟to do形式，如：
be glad to do.
be pleased to do
be happy to do
be sorry to do
be important to do
be easy to do
be difficult/hard to do
……
I am glad to meet you.
It is hard to answer this question.
It is important to learn a fireign language well.
