不定式结构和它所修饰的名词是主动关系
即 名词是不定式结构的逻辑主语
He was the last guest to arrive. (=who arrived)
Have you got a key to unlock the door. (=that can unlock the door)
不定式结构与它所修饰的名词是动宾关系
即 名词是不定式结构的逻辑宾语
He has a large family to support. (=that he must support.)
This is the best book to read. (=that we can read.)
若不定式属于及物动词，或由于意义或结构上的需要，其后须加适当的介词。
Mary needs a friend to play with.
不定式结构与它所修饰的名词有时是一种同位关系。
I have no wish to quarrel with you.
He hit back the urge to tell a lie.
attempt, chance, effort, freedom, intention, reason 等名词后，可用不定式/介词+ing，意义无差别
They made an effort to finish/ at finishing the work in one day.
ability, agreement, ambition, anxiety, disposition意向，obligation, refusal等名词后，一般是不定式
He is under no obligation to do that.
aptitude资质, delay, excuse, interest, genius, hope, skill等名词之后，通常是介词+ing形式
He had a great aptitude for inventing new things.
