could和can的区别

　　could的例句：

　　I could come next week, or the week after.

　　我可能下周来，或者再下一周。

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

　　这次手术可使他多活两三年。

　　Events could have developed in a dramatically different way.

　　事情本来可以发展成另一种样子。