◆他能通过考试。
误：He can be able to pass the exam.
正：He can pass the exam. / He is able to pass the exam.
析：be able to 不能与 can 用在同一个谓语中。
◆这工作可按时完成。
误：The work is able to be finished in time.
正：The work is capable of being finished in time.
正：The work can be finished in time.
析：按规范语法：be able to之后一般不接动词的被动式，尽管在 现代英语中已有人打破此规则，但初学者仍以不用为宜。
◆他在公共汽车后面追，并追上了。
误：He ran after the bus, and could catch it.
正：He ran after the bus, and was able to catch it.
析：表示过去特定情况下的能力，可用 was (were) able to 而不 能用 could，不过这种差异在否定句中便不存在了。如：He ran after the bus but wasn’t able to [couldn’t] catch it.