动名词作定语往往表示被修饰词的某种用途。如：
a walking stick =a stick for walking=a stick which is used for walking
a washing machine=a machine for washing=a machine which is used for washing
a reading room=a room for reading=a room which is used for reading
a measuring tape=a tape for measuring=a tape which is used for measuring
sleeping pills=pills for sleeping=pills which is used for sleeping
动名词作定语时，在句子中通常有两个位置：如果是单个的动名词作定语，常放在被修饰的名词前面作前置定语;如果是动名词短语作定语，则放在被修饰名词的后面，作后置定语。
1.动名词作定语，说明被修饰词的性质、特征或用途。如：
He may be in the reading room.
他可能在阅览室里。
They set up an operating table.
他们搭起一个手术台。
2.动名词作定语，表示其与被修饰词之间为主动关系，并且表示动作正在进行。如：
There are about 200 students studying in this school.
大约有200个学生在这个学校学习。
Who is the woman talking to our English teacher?
正在和我们英语老师谈话的那个女人是谁?
3.有些动名词已经转化成了形容词，常用作定语修饰物，表示“令人……”：exciting, shocking, puzzling, confusing, disappointing, discouraging, boring, tiring, moving, touching, interesting, satisfying, frightening, amazing等。如：
That must be a terrifying experience.
那肯定是一次可怕的经历。
The experiment was an amazing success.
那实验是一个惊人的成功。
注意：如果动名词与被修饰词之间是被动关系，就要用它的被动式being done，通常表示正在被做，常作后置定语。如：
The tall building being built now is our new school.
正在被修建的那建筑是我们的新学校。
That question being discussed was presented by the headmaster.
正在被讨论的那个问题是校长提出来的。