could和can的區別

　　could的例句：

　　I could come next week, or the week after.

　　我可能下周來，或者再下一周。

　　The operation could prolong his life by two or three years.

　　這次手術可使他多活兩三年。

　　Events could have developed in a dramatically different way.

　　事情本來可以發展成另一種樣子。