助动词 (Modal Auxiliaries)
1.语气助动词，给予主动词一种感情的色彩，如许可、禁止、猜测、可能、潜力等。
2.助动词 + V ; 助动词 + not + V; 助动词 + S + V ?
can, could, be able to
能力
Michael can speak French. Michael会说法文。
Can he (=Is he able to) speak German, too? 他也会说德文吗？
I could (=was able to) speak a little Spanish.
我以前会说一点点西班牙文。
I can't solve this math problem. 这道数学题我解不出来。
Cats are able to climb trees. 猫会爬树。
What can I do for you? 有何贵干？
I could (=was able to)run fast when I was young.
我年轻的时候跑得很快。
She spoke pretty fast, but I could (=was able to) understand what she was saying.
她说话相当快但我了解她说什么。
I couldn't (=wasn't able to) sleep well last night.
我昨晚睡不好。
