①can和be able to都表示能力,意思上没多大区别.但can只有现在和过去时,而be able to则有更多的形式.但当成功地完成某一具体动作时,通常不用could而用was/were able to来表示.这时was/were able to 相当于managed to,表示经过一番努力,终于能够完成某事.如:
Can you use chopsticks
The wounded man still was able to get to the village and was saved in the
end.
can和could都可以表示能力,技能,许可,建议或请求和可能性.但比较委婉客气地提出问题或陈述看法,一般用could,回答时则用can.如:
Could you help me carry the bag
Can I help you
参考资料:http://www.pkuschool.com/ask/q.asp?qid=80434