spend的主语必须是人， 常用于以下结构：(1) spend time ／money on sth. 在……上花费时间（金钱）。例：I spent two hours on this maths problem. 这道数学题花了我两个小时。(2) spend time ／ money (in) doing sth. 花费时间（金钱）做某事。例：They spent two years (in) building this bridge. 造这座桥花了他们两年时间。(3)spend money for sth. 花钱买……。例：His money was spent for books. 他的钱用来买书了。
pay的基本用法是：(1) pay (sb.) money for sth. 付钱（给某人）买……。例：I have to pay them 20 pounds for this room each month. 我每个月要付20英磅的房租。(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?蒺t worry!I'll 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? I'll pay it back next week. 你能借给我12块钱吗？下周还你。（6）pay off one's money还清钱