改英语句型John needs to cut his hair because his hair is too long

改英语句型
John needs to cut his hair because his hair is too long.
John needs to_____his hair ______because of his long hair.
After planting the trees you should water them often.
The trees ______ _____ _____ often after they _____ _____.
We allow you to use the computers.(改为被动句)
You _____ _____ to use the computers.