look down：向下看,不带宾语,如：Don't look down while you are on the edge of the cliff.在悬崖边上别往下看.
look at sb./sth.：看着（某人/某物）,如：Look at my eyes and say it again.看着我的眼睛再说一遍.
look down at sb./sth.：低头看（某人/某物）,如：She looked down at her toe and murmured something.她低头看着自己的脚趾,小声嘟哝着.
look down on/upon：看不起,小看（某人）,如：You shouldn't look down on someone else because he is poor.不要因为别人穷而看不起他.