一、使役动词+宾语
在let, make, have等使役动词接宾语的结构中，通常用不带to的不定式。但当使役动词为被动态时，不带to不定式转为带to不定式。
主动：Let’s go and play basketball. 让我们去打篮球吧！
The teacher made him stay there quietly. 老师让他安静地呆在那里。
Her father had her do everything. 她爸爸让她做所有事。
被动：He was made to stay there quietly. 人家让他安静地呆在那里。
She was let to tell everything. 她被迫说出一切。
二、感官动词+宾语
在see, hear, observe, notice, feel, watch等感官动词接宾语的结构中，用不带to不定式。但是当以上动词为被动态时，不带to不定式转为带to不定式。
主动：I could only see him go. 我只能看着他走。
Mother watched him eat his breakfast. 妈妈看着他吃完早饭。
We felt the house shake. 我们感到房子的震动。
被动：He was seen to enter the house. 有人看到他进入这个房子。
Someone was heard to open the door by us. 我们听到有人开门。
三、have known+宾语
“have known+宾语” 后用不带to的不定式，意思是“看过、听过”，等于“have seen”或“have heard”。同样，若是被动态，也需要加to。
主动：We have never known the boy smile.我们从来不知道那个男孩会笑。
被动：That boy has never been known to smile. 没有人见过那个男孩笑。
不定式有to还是没有to？什么时候带to？什么时候不带to？什么时候既可以带to又可以不带to？这些你都get到了吗？
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