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。

They let the strange go.

-> The strange was let go.

2）若let 后宾补较长时，let 通常不用被动语态，而用allow或permit 代替。

The nurse let me go to see my classmate in the hospital.

-> I was allowed / permitted to see my classmate in the hospital.