be used for "为...而用"eg: This pen is used for writing.be used as "被当作...来用"eg: These pictures are used as wallpapers.be used by "由...使用"eg:These books are used by teachers.因为for/as/by都是介词，所以后面可以接名词、代词或动名词。
be used for "为...而用"eg: This pen is used for writing.be used as "被当作...来用"eg: These pictures are used as wallpapers.be used by "由...使用"eg:These books are used by teachers.因为for/as/by都是介词，所以后面可以接名词、代词或动名词。
