spend的主語必須是人
cost的主語是物或某種活動,還可以表示“值”
pay的主語一般是人pay的主語一般是人,pay---paid---paid,基本用法是：(1) pay +名（ 人）+名（款）+（for名） == pay +名（款）+to名（人）+（for 名）為了（某事,物）付（款）給（某人）.例：Did you pay him 100 dollars for that old bike?==Did you pay 100 dollars to him for that old bike?你付給他100 美元買他那輛舊自行車了嗎?(2)pay for sth.付……的錢.例：I have to pay for the book lost.我不得不賠丟失的書款.(3)pay for sb.替某人付錢.例：Don’t worry!Ill pay for you.別擔(dān)心,我會給你付錢的.(4)pay sb.付錢給某人.例：They pay us every month.他們每月給我們報酬.(5)pay money back 還錢.例：May I borrow 12 yuan from you?Ill pay it back next week.你能借給我12塊錢嗎?下周還你.(6)pay off ones money還清錢.例：I’ll pay off my money to you soon!我會盡快還清我的錢!
buy (1)用作動詞,表示“買”,可帶雙賓語；若雙賓易位,通常要用介詞 for.例：我叔叔為我買了本英語詞典.正：My uncle bought me an English dictionary.正：My uncle bought an English dictionary for me.(2)表示向某人買某物,通常用介詞 from.例：He bought this watch from a friend.他從一位朋友那兒買了這塊手表.(3)表示用多少錢買某物,通常用介詞 for.例：I bought the computer for $600.我花600美元買了這臺電腦.He bought it for very little money.他買它只花了一點點錢.【注】有時也用介詞 at,側(cè)重指價格.例：He bought them for [at] 10p each.他以每個10 便士的價格買下了它們.(4)是終止性動詞,因此在肯定句中不宜與一段時間連用.例：這房子我已買了十年.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.誤：I have bought this house for ten years.