can的过去式英文单词

　　Could you forward any mail to us in New York?

　　你能不能把所有信件转寄到纽约给我们？

　　They could not ensure a smooth transfer of political power.

　　他们无法保证政权的顺利交接。

　　I could come next week, or the week after.

　　我可能下周来，或者再下一周。