我五岁的时候就能自己系鞋带了。

When I was five, I ___could tie my shoes by myself.

你明天可能赶不上火车，最好早点出发。

You ___could miss the train tomorrow, so you'd better leave early.

能请你帮我拿一下那个文件夹吗？

___Could you help me grab that folder, please?

如果你早点起床，就能赶上早班车。

If you woke up earlier, you ___could have caught the early bus.