使役动词后不定式符号to的省略问题
第一种情况：当不定式用于let, make, have等使役动词后作宾语补足语时，不定式必须省略to。如：
Let me give you a piece of advice. 让我给你一个忠告。
He had his son clean the car. 他叫他的儿子擦车。
They made me repeat the whole story. 他们非让我把整个事件再说一遍。
但是，当使役动词用于被动语态时，其后的不定式则必须要带to。如：
虽然他以前经常把他小妹妹弄哭，但今天他被他小妹妹弄哭了。
误：Though he had often made his little sister cry, today he was made cry by his little sister.
正：Though he had often made his little sister cry, today he was made to cry by his little sister.
注意，表示使役意义的let和have很少用于被动语态。
第二种情况：表示“使”“迫使”的force, get, oblige等动词后接不定式作宾语补足语时，不定式必须带to（不能省略）。如：
They forced her to sign the paper. 他们强迫她在文件上签字。
The police obliged him to leave. 警方强迫他离开。
Once she has decided on doing it, you won’t get her to change her mind. 一旦她下决心干，你就别想使她改变主意。