when are trees often planted by people改为主语动态

when are trees often planted by people改为主语动态 when are trees often planted 英语语法解答1.When are trees often planted? ,为什么"planted"要用过去式.2.W when do people often plant trees?改为被动语态 mang trees have been planted by people the There are many trees planted Some trees weren't planted by people last year.不是否定句中用any吗,怎 Look!Some trees _____ by them.A.are planted B.are planting C some seeds are planted on purpose by people to grow flowers, There are some trees by the river.改为否定句 twenty trees had been planted by the time we left they planted trees last year (改为一般疑问句）