下面在附加一些相关知识，希望对你有帮助
感官动词和使役动词后面的不定式不加to.
感官动词，如hear, see, notice, feel, watch等
使役动词如：make, let, have（要、叫、使）等，后面不加to.
help后面的动词不定式可加可不加to.
eg.
1. Professor Wang let his assistant leave early enough to catch the last bus.
王教授让他的助手早走，好赶上末班车。
2. I heard Mary cry.
我听见玛丽哭了。
3. We watched them play football.
我们观看他们踢足球。
注意：这些词变为被动语态时，一律加to.
例：The boss made him work.
老板迫使他干活。
He was made to work （by the boss）
使役动词
http://www.jichuyingyu.com/Article/grammar/200604/44.html(flash动画教程)
使役动词是表示使、令、让、帮、叫等意义的不完全及物动词,主要有make(使,令),let(让),help(帮助),have(叫)……
使役动词:比如make,have,let,order.....
所谓使役,就是使什么怎么做.
比如I make the boy cry.我让那个男孩哭了.I have the girl wait for me.我让那个女孩等我.
总之,使役就是让什么怎么样,或怎么做.
使役动词有以下用法：
a. have somebody do sth让某人去做某事
??i had him arrange for a car.
b. have somebody doing sth.让某人持续做某事。
??he had us laughing all through lunch.
注意：用于否定名时，表示“允许”
i won't have you running around in the house.
我不允许你在家里到处乱跑。
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小议“使役动词”的用法
1. have sb do 让某人干某事
e.g:What would you have me do?
have sb/sth doing 让某人或某事处于某种状态，听任
e.g: I won't have women working in our company.
The two cheats had the light burning all night long.
have sth done 让别人干某事，遭受到
e.g:you 'd better have your teeth pulled out.
He had his pocket picked.
notes: "done"这个动作不是主语发出来的。
2.make sb do sth 让某人干某事
e.g:They made me repeat the story.
What makes the grass grow?
notes: I was made to repeat the story.
make sb/sth done/adj./n
e.g. The news made him happy.
He couldn't make himself heard above the noise of the traffic.
His actions made him universally respected.
He made her his wife.
3.get sb to do 使某人干某事
e.g: I can't get anyone to do the work properly.
get sth done 让别人干某事
e.g: I must get my hair cut.
Can you get the work finished in time?
4.leave sb to do sth 让某人干某事
e.g: We left him to paint the gate.
I'll leave you to settle all the business.
leave sb doing 让继续处于某种状态
e.g: Don't leave her waiting outside in the rain.
We left him painting the gate.
leave sth done/adj./prep.phrase
e.g:Please excuse me if I have left any of your questions unanswered.
His illness has left him weak.
I was left with a ray of hope.