spend的主语必须是人
cost的主语是物或某种活动,还可以表示“值”
pay的主语一般是人pay的主语一般是人,pay---paid---paid,基本用法是：(1) pay +名（ 人）+名（款）+（for名） == pay +名（款）+to名（人）+（for 名）为了（某事,物）付（款）给（某人）.例：Did you pay him 100 dollars for that old bike?==Did you pay 100 dollars to him for that old bike?你付给他100 美元买他那辆旧自行车了吗?(2)pay for sth.付……的钱.例：I have to pay for the book lost.我不得不赔丢失的书款.(3)pay for sb.替某人付钱.例：Don’t worry!Ill pay for you.别担心,我会给你付钱的.(4)pay sb.付钱给某人.例：They pay us every month.他们每月给我们报酬.(5)pay money back 还钱.例：May I borrow 12 yuan from you?Ill pay it back next week.你能借给我12块钱吗?下周还你.(6)pay off ones money还清钱.例：I’ll pay off my money to you soon!我会尽快还清我的钱!
buy (1)用作动词,表示“买”,可带双宾语；若双宾易位,通常要用介词 for.例：我叔叔为我买了本英语词典.正：My uncle bought me an English dictionary.正：My uncle bought an English dictionary for me.(2)表示向某人买某物,通常用介词 from.例：He bought this watch from a friend.他从一位朋友那儿买了这块手表.(3)表示用多少钱买某物,通常用介词 for.例：I bought the computer for $600.我花600美元买了这台电脑.He bought it for very little money.他买它只花了一点点钱.【注】有时也用介词 at,侧重指价格.例：He bought them for [at] 10p each.他以每个10 便士的价格买下了它们.(4)是终止性动词,因此在肯定句中不宜与一段时间连用.例：这房子我已买了十年.正：I bought this house ten years ago.正：I have had this house for ten years.正：It’s ten years since I bought this house.==Ten years has passed since I bought this house.误：I have bought this house for ten years.
