若能互换,则无区别.
下面说说定语从句和不定式可以互换的条件.
首先,要清楚动词不定式用于表示尚未发生的动作（相对于谓语动词）.如：
She has lots of homework to do tonight.(作业还没做）
Have you got something to eat?（这些东西还没被吃掉）
这就意味着,不定式做定语时,它所表达的这个动作尚未发生.
要想定语从句和不定式互换,
条件之一：定语从句表达的动作应该是现在时或将来时（即相对于主句的谓语,它还没发生）.
条件之二：先行词在定语从句中充当主语或宾语.
如：句子：Mary今晚有许多作业要做.
Mary has lots of homework which has to be done tonight.
（定语从句表达的作业还没做；homework在从句中当主语）.
Mary has lots of homework (which) she has to do tonight.
（定语从句表达的作业还没做；homework在从句中当宾语）.
这两句话都符合上述两条件,可以用动词不定式代替：
Mary has lots of homework to do tonight.
又如：
The car which he is going to buy is for his lover instead of his wife.
= The car to be bought is for his lover instead of his wife.
但是,
The car which he bought yesterday is for his lover instead of his wife.就不能用不定式来替代了.因为车已经买了.
即使符合上述条件,尽管在语法上成立,出于行文的考虑,并不是定语从句和不定式都可以随便对换,这关系到行文的约定俗成和句子的简繁.
