1）当let后只有一个单音节动词，变被动语态时，可用不带to 的不定式。例如：
They let the strange go.他们放陌生人走了。
——> The strange was let go.
2）当let 后宾补较长时，let 通常不用被动语态，而用allow或permit 代替。例如：
The nurse let me go to see my classmate in the hospital. 那护士让我去探望住院的同学。
----> I was allowed / permitted to see my classmate in the hospital.