1.can的過去式是could，但could 除了可作can的過去式外，還可表比can更委婉的語氣，因此在回答由could 表示請求的問句中，不能用could，但可用can。如： —Could you lend me a hand? —Of course I can.
2. 在表示現在或過去的智力、體力、技能等方面的“能力”時，can, could 常可與be able to互換。如：He could (or was able to) swim five miles when he was a boy.
3. be able to有比can更多的形式，如"will be able to"，"have (has) been able to" "had been able to" 等等。因此be able to常被用來表示can所不能表示的將來或完成的概念。如： They will be able to run this machine on their own in three months. 他們過三個月就能自己開這臺機器了。
4. 在表示成功地做了某事時，不用could，而用was / were able to (或managed to, succeeded in doing sth )來表示。如： He was able to (or managed to) finish the job yesterday。 但是couldn"t和wasn"t / weren"t able to 常可互換。如： I couldn"t / wasn"t able to finish the job yesterday.
5. 與感覺動詞連用時，can，could亦可與be able to互換。如： We could/were able to smell something burning in the kitchen.
1.can的過去式是could，但could 除了可作can的過去式外，還可表比can更委婉的語氣，因此在回答由could 表示請求的問句中，不能用could，但可用can。如： —Could you lend me a hand? —Of course I can.
2. 在表示現在或過去的智力、體力、技能等方面的“能力”時，can, could 常可與be able to互換。如：He could (or was able to) swim five miles when he was a boy.
3. be able to有比can更多的形式，如"will be able to"，"have (has) been able to" "had been able to" 等等。因此be able to常被用來表示can所不能表示的將來或完成的概念。如： They will be able to run this machine on their own in three months. 他們過三個月就能自己開這臺機器了。
4. 在表示成功地做了某事時，不用could，而用was / were able to (或managed to, succeeded in doing sth )來表示。如： He was able to (or managed to) finish the job yesterday。 但是couldn"t和wasn"t / weren"t able to 常可互換。如： I couldn"t / wasn"t able to finish the job yesterday.
5. 與感覺動詞連用時，can，could亦可與be able to互換。如： We could/were able to smell something burning in the kitchen.