有一读者问这样一句话：
(1)At that time we lacked words with which to express our thanks.
请问如何分析with which to express our thanks? 这是不是一个定语从句？ 为什么？
这里的with which to express our thanks可以唤作不定式特殊结构，也可以唤作不定式从句。它在此显然用作定语，修饰其前的名词words。这种“with which+带to 的不定式”结构在英语里并不罕见，再如：
(2)I must buy a hammer with which to nail down the box.
(3)The factory has bought a new machine with which to make the spare parts.
(4)This is a good instrument with which to measure vibration.
除with外，还有一些介词也可以后接“which+带to的不定式”，如：
(5)They used it as a subject about which to quarrel among themselves.
(6)The seats have no back against which to lean .
(7)This is the best means by which to gain the end.
(8)We would look for water and something from which to make a fire to cook with.
(9)A notebook is a book in which to write notes.
(10)It's a board on which to cut vegetables.
这种结构其实也并不限于which,whom亦可用于这种结构，如：
(11)She is not a person on whom to rely .
“介词+which（或whom）+带to的不定式”结构之后如接较短的词语，亦可简化为“带to的不定式+介词”结构，如上述例（8）中的to cook with 即是with which to cook 的简略形式（这里用to cook with 显然是由于其前已用了from which to make结构）。同样，例（4）也可简化为：
(12)This is a good instrument to measure vibration with .
例（11）也可简化为：
(13)She is not a person to rely on .
在历史上，“with which （或whom）+带to的不定式”结构的出现要晚于“带to的不定式+
介词”结构，但前者较后者更接近于口语。
上述两种结构与我们通常所说的定语从句（即“关系代词+主语+谓语”结构）有对应的关系。比如上述例（4）或（12）即相当于：
(14)This is a good instrument with which they measure vibration. （或which they measure vibration with）
同样，上述例（11）或（13）亦相当于：
(15)She is not a person on whom you may rely . （或whom you may rely on）
由此可以看出，上述用作定语的不定式结构也就相当于一定语从句，因而也就可以叫做不定式定语从句了。同时，却也使我们感到，不定式定语从句不及“关系代词+主语+谓语”定语从句那么明白通俗。
