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
