不定式作定语表示动宾或介宾关系时，其实是定语从句的简略。例如：

...would leave him nothing (that) he is to run his business with.

你可以看出，关系代词that是句末的介词with的宾语，而that指代nothing。没有句末的介词，关系代词that就无成分可做，为错误。

I have my little bother to look after.

= I have my little brother who(m) I am to look after.

I need a knife to cut the watermelon with.

= I need a knife which I am to cut the water melon with.

介词提前则which不可省略：

I need a knife with which I am to cut the watermelon.

= I need a knife with which to cut the watermelon.

以上定语从句省略划线部分就得到不定式作定语，表示动宾关系或介宾关系。