can的过去式英文

　　I could come next week, or the week after.

　　我可能下周来，或者再下一周。

　　How long can a human being go without sleep?

　　人不睡觉能活多久？

　　The transition from school to work can be difficult.

　　从在学校念书到上班工作，这一转变有时候可真不容易。