e able to do sth 有能力做某事,可以用于各种时态,往往表示一次性的能力,或是在特定的情境中能够做某事.
出题时,经常和can do sth放在一起进行区分.can do sth表示一般能力,可以用于一般过去时态.
如：When the fire took place, all the workers were able to run away.这句里不能用could ran away.
The little boy could read.那小孩会认字了,不用is able to read.