(二) 不定式
A. 动词不定式的省略
①、help后面可以省略to
help to do
help sb. to do
例：A good exercise program helps teach people to avoid
A B
the habits that might shorten the lives.
C D
分析：D错，应改为their. 定冠词在句中不如代词所有格明确，如果lives后面有定语就可以用the。注意（A）并没有错，help后面可以接不带to的不定式。
②、使役动词后面要省略to, 这样的动词只有三个
make,
let,
have sb. do sth
注意get不是使役动词，get sb. to do sth.
例：A conductor used signals and gestures to let the musicians
A B
to know when to play various parts of a composition.
C D
分析：let是使役动词，后面不需要to, C改为know
③、感官动词
hear, feel, watch, see, notice, observe
see sb do sth. 强调过程
see sb doing sth 正在做某事
B. 动词不定式的固定用法
(1)表示第一人
the first woman to do sth.