can和could是我们最常用来表达”可以“的意思，而且之前的文章有讲过can和could分别是使用于现在时态和过去时态，那除了这两点，是否还有其他区别？它们和be able to 又有什么关系？
Can
我们用can表达事情是可能的，或者某人有能力做某件事情。表达的结构为：
can+动词（can do/ see/ eat......）
- We can see the park from our living room.
我们可以从客厅看见那个公园。
- He can speak four languages.
他可以说四种语言。
有时候我们也会用be able to 表示”可以“，但是can 更常用一些，不过在现在完成时中，都是用be able to 来表示。
- Tom can't sleep.（现在时态）
Tom 无法入睡。
- Tom couldn't sleep.（过去时态）
Tom 无法入睡。
- Tom haven't been able to sleep recently.（现在完成时态）
Tom最近无法入睡。
Can的否定形式是：can not/ can't，表示”不能“。
- I can't come to the party on Saturday.
周六的派对我去不了。
同样，can't 也可以表示”觉得某事不可能“
- You can't be hungry, you've just had lunch.
你不可能饿了，你才刚吃完午饭。
而表示”确定某事是真的“，则使用must。
- John: My house is very near the station.（我的房子非常靠近车站）
Mike: Oh, it must be very noisy.（噢~那肯定很吵）
Could
之前有讲过，could是can 的过去时态，用于过去式的句子中，同样表示”可以“。它经常跟see，hear，smell，taste，feel，remember，understand一起使用：
- He spoke in a very low voice, but I could still understand what he said.
他用很小的声音说话，但是我还是能够明白他的意思。
- When we went into the house, we could smell the buring from the kitchen.
当我们走进房间的时候，可以闻到从厨房传来的烧焦味。
除了作can的过去式使用，could也可以表示”可能“，类似于may/ might。
- The phone is ringing, it could be Jill.
=The phone is ringing, it may/ might be Jill.
电话响了，可能是Jill打来的。
- The weather is nice now, but it could change later.
=The weather is nice now, but it may/ might change later.
现在天气很不错，但是一会可能会变天。
但是，couldn't 是不同于may not和might not的。
- She couldn't have seen you, because she was too far away.
她不可能看到你，因为距离太远了。
- She might not have seen you.
她可能没有看到你。
疑问句中的Can/ Could/ Would...?
在表达”请求“”提出要求“”许可“”邀请“的问句，通常分别使用Can，Could，Would。
请求别人做某事时，经常使用can或者could：
- I wonder if you could help me.
- Can/ Could you wait a moment, please?
- Can you do me a favour?
- Could you tell me how to get to the station?
想要某些东西的时候，可以使用Can I have.../ Could I have...：
- Can I have these stamps, please?
我可以拿这些邮票吗？
- Could I have a cup of tea, please.
我想要一杯茶
在请求许可时，可以使用 Can，Could，May：
- Can I speak to Tom, please?
- May I come in? --Yes, please.
允许别人做某事时，最常用Can：
- Yes, you can use the phone.
是的，你可以用这部电话。
主动提出想要做某事时，可以用Can I...?：
- Can I help you?
- Can I give you something to drink?
邀请别人时，我们可以用Would you like...?：
- Would you like to come to dinner tonight?
--"Yes, I'd love to."
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