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.
From:https://www.zgghmh.com/CET-6/20007629.asp