doing / done / to do
相当于名词，作主/宾/表语 doing / to do
Fishing is my favourite sport.
It is my favourite sport fishing with my friends every week. it作形式主语
I often fish for hours without catching anything.
Both men started running through the trees.
My dream is travelling around the world.
To fish is my favourite sport.
It is my favourite sport to fish with my friends every week. it作形式主语
Both men started to run through the trees.
My dream is to travel around the world.
修饰名词/句子，作定语/状语 doing / done / to do
to do 目的/即将去做
Looking around, I realized that I was the only passenger on the bus.
Certain jobs have gone away for good, Replaced by machines.
I sat in the front of the bus to get a good view of the countryside.
My aim, to become a millionaire by the time I'm 30, seems impossible.
doing / being done
He was surprised at discovering other people there.
He was surprised at being discovered there.
doing / having done
Doing his homework, the little boy watched TV.
Having done his homework, the little boy watched TV.
to do / to be done
The man sat there to ask the policeman about what had happened.
The man sat there to be asked about what had happened.