He plants flowers every year.
--> Flowers are planted by him every year.
We speak Chinese.
--> Chinese is spoken by us.
I look after the old man.
--> The old man is looked after by me.
Do you clean your room every day?
--> Is your room cleaned by you every day?
Tom doesn’t wash his shoes every week.
--> Tom's shoes aren't washed by him every week.
I wrote a letter yesterday.
--> A letter was written by me yesterday.
Mary made 3 kites the other day.
--> 3 kites were made by Mary the other day.
Did they build this bridge last year?
--> Was this bridge built by them last year?
They asked us to sing a song.
--> We were asked to sing a song by them.
They didn’t water the flowers.
--> The flowers weren't watered by them.
参考资料：http://zhidao.baidu.com/question/402783241