can与could
1）． can
（1）表示体力或脑力方面的“能力”，也能表示根据客观条件能做某事的“能力”。
He can drive.
Can you jump as far as he did?
I can’t catch up with Jim.
can与be able to二者都表示“能够”，区别是：can只有现在时和过去式，而beable to可用于各种时态。如：
I’ll be able to speak French in half ayear.
He hasn’t been able to finish it on time.
（2）表示允许：
You can go now.
Can I use your bike?
（3）表示推测：can表示猜测或怀疑只用于疑问句及否定句中。
Who can it be?
It can’t be true.
2）． could
（1）can的过去式，表示过去有能力及过去的可能性。
He couldn’t climb up the mountain.
The news could be true.
（2）表示客气地请求或委婉地陈述意见。
Could you go skating with me tomorrow?
I’m afraid I couldn’t give you an answertonight.
（3）表示惊异、怀疑、不相信等态度
How could he say that? He couldn’t be in Beijing.
