able

用法

be able to-V表示can V之意,而以生物为主词时, can的过去式、未来式及完成式可分别用下列方式来表达:

was [were] able to

will [shall] be able to

have [has, had] been able to

尤因could常用来表示现在或未来的条件,故欲叙述单纯的过去事实时,以使用was[were] able to较为妥切:

I was able to help you yesterday.

(我昨天能帮你的忙。)

I could help you.

(我或许能帮你忙。)

补充

be able to-V的主词通常为生物,故无生物应避免使用,如:

A single spark is able to start a prairie fire.

应改为

A single spark can start a prairie fire.

(星星之火可以燎原)。

但在科技文体中则常见及以无生物为主词:

Heat is able to cure breast tumors in some animals.

(高温可以治疗某些动物的乳房肿瘤。)

字辨

able: 指具有必要的能力

I am able to solve the problem.

capable: 指具有做工作的能力;可指人,亦可指物

a capable workman

competent: 表示具有胜任某种工作的足够能力

a competent secretary