1.某人花费多长时间做某事：It takes sb.+一段时间 to do sth.(主语经常是it,后接真正的主语不定式 )
如：It often takes me an hour to finish my homework.
It took him two days to solve the problem.
2.某人花多少钱买某物：Sb.spend +钱的数目on sth.（主语必须是人）
某人花多少时间做某事：Sb.spend +一段时间 in doing sth.(in 可省略)
Mary spent 50 dollars on the dress yesterday.
They spent a week (in) finding out the truth.
3.用cost时,主语必须是物.
The book cost me ten yuan.
4.Pay 一般指付钱,主语必须是人.(pay for sth./ pay sb.+钱的数目 for sth.)即直接带人作宾语时,不跟介词； 带物作宾语时要跟介词for.
He couldn't pay for the meal because he had little money with him.
I paid the girl 20 yuan for the picture.