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关于陈老师针对我的解答写的“补充说明”，我都认真看过。我发现网友都特别支持她，点赞的40多个，对此我感到由衷的高兴！我们“答疑网”有这样一位资深的高水平的老师，真是英语网友们的幸运！

在这里我不想多说了，我的观点在那里摆着，而且不会轻易改变的。大家如果有耐心的话，可以再看一遍。

首先说明一点，我并没有完全否认陈老师的解答——我开始的第一句话是：“我不完全同意陈老师的解答”，有这样几层意思，我再重复一下：

1.  able 指人的“能力”，原则上“人”作主语是非常容易理解的。如果“物”作主语，从逻辑上讲，甭说是被动语态，即便是主动语态也不合乎常理。我们不会说：The meeting is able to take place this afternoon. 更何况说The meeting is able to be held this afternoon.

所以，葛传槼先生说这个句子是错的：The mistake is able to be corrected.  我一开始就赞同。

2.  can 和be able to 有着较多的区别，我在前面已经提到。正是因为这些差别，才出现了不同的用法。当表示“推测”和“许可”时，be able to 无法替代它。因此，以下两句只能用can，不能用be able to，更不用说是被动了。

The baby can't be hungry. I fed him with milk just now. （表推测）

这孩子不可能饿了，我刚喂了他牛奶。

You can use my computer this morning. （表许可）

你今天下午可以使用我的电脑。

3.  反过来，could 虽然表示“(过去)有能力做某事”，但不表示“结果”，而was able to 除了表示“(过去)有能力做某事”外，还有一层重要的意思，就是表结果：“设法做了某事(managed to do sth.)”，“成功地做了某事(succeeded in doing sth.)”。以下两句都是被动语态，其中的be able to 是不能用could 代替的。

The fire was very big, but most people were able to be rescued. (不用could)

火势很大，但多数人还是能够被救了出来。

She was able to be persuaded by me though she was stubborn. (不用could)

虽然她固执，但还是能够被我说服了。

4.  我的解答，并非全盘否认。英语前辈们总结出的规则应该说没有错，但不够完整、不够全面的情况，也是存在的，所给的例句只是反映某个侧面，而且没有说明“这是为什么”。好了，我的解答到此为止，谢谢陈老师和网友们的关注。（2015,11,11 晚）

网友提的这个问题，还真是一个问题。

▲我不完全同意陈老师的解答。我个人认为，able 这个词是指“人”的能力。也就是说，它的主语一般应该是“人”，而不是“物”。从这一点考虑，只要人作主语，那么be able to 后面的不定式，用主动或被动应该都可以。但是，如果说Something is able to be done. 则似乎讲不通，原因是主语不是“人”，而是“物”。这是从逻辑方面判断和理解的。例如：

可以说：I am able to correct the mistake.

但The mistake is able to be corrected. 似乎就讲不通。

但是，以下句子都可以说：

She was able to be persuaded by me though she was stubborn.

虽然她固执，但还是能够被我说服了。

Make sure that you are able to be seen by cars when crossing the street.

确保你过马路时能够被汽车看到。

▲我也查阅了《葛传椝英语惯用法词典》（2012年版）和《牛津英语用法指南》（第3版）的有关用法。葛传槼教授的例句The mistake is able to be corrected. 跟Michael Swan 的例句不一样，后者的例句主语仍然是“人”。 《牛津英语用法指南》并没有说He’s not able to be understood. 不正确，而是说：He can’t be understood. 比He’s not able to be understood.自然。

此外，我输入able to be，查阅了美国语料库Corpus of Contemporary American English (COCA)，得到的结果是：1309例。

其中，“able to be + 表语”占了一定的比例；而“able to be +过去分词”（被动结构）也占了很大的比例。（1309例一共14页，每页100例。以下是从第1页100个例句范围内摘录的，约占40%）由此可以看出：be able to +不定式的被动式，在现实语言中十分常见。

现摘录部分如下：

the three bruises which were able to be seen on the right side of the scalp.

to be able to be put in the position of the federal government

a system there to be able to be inserted

They're not going to be able to be erased

And instead of being able to be presented with both sides of the story, it's very polarized

This, you know, notion of gift cards being able to be used

you truly believe in and being able to be balanced

unemployed young men who are able to be manipulated by either

I think that that choice should at least be able to be discussed and open to them.

I feel my community has been able to be recognized finally

I designed it in such a way that it would be able to be used for the prep and the finishing of food

students with any missing data beyond the first two measurement occasions were able to be included in our observation models

that their PWS group was able to be subdivided on the participants' pretreatment assessment

Lots of great jokes will be able -- were able to be made at her expense through the night

before families are able to be notified

they are - never be able to be traced back.

I mean, things that would never have been able to be played on the radio

she shouldn't able to be enriched by it

Able to be exploited along with all the rest

for that to be able to be translated into the product

Being able to be coached by a college coach got us ready for the next level

and able to be manipulated by its smaller, stronger neighbors

what was once only able to be given to us through paper

The schools under other countries are able to be developed or revised in accordance with

while the two patients with lateral incisions and soft tissue infection were able to be treated with oral antibiotics

The survey was designed to be able to be completed within a single 50-minute class period

which must be able to be put into phrases

beta-blockers may be able to be initiated before ACE inhibition

their hybrid condition or cultivation is able to be remarked upon

▲有一个问题需要注意：could表示“(过去)有能力做某事”，但不表示“结果”，而was able to 除了表示“(过去)有能力做某事”外，还有一层重要的意思，就是表示结果：“设法做了某事(managed to do sth.)”，“成功地做了某事(succeeded in doing sth.)”。例如：

 She was able to be persuaded by me though she was stubborn. 虽然她固执，但还是能够被我说服了。

这句话如果改为：She could be persuaded by me though she was stubborn. 反而不正确。

【个人观点如下】

由于be able to 只表示“能力”，而can 除了表示“能力”之外，还可表示“推测”和“许可”，所以，can be done 的用法，语境广泛，限制较小；而be able to be done 的用法，限制相对较大。

1. 当句子主语为“物”时，应避免在be able to 后使用被动结构。

2. 当句子的主语为“人”时，可以在be able to 后使用被动结构。

3. 当表达过去“设法做了某事”时，应该用was/were able to be done，而不用could be done。例如：She was able to be persuaded by me though she was stubborn. 不说 She could be persuaded by me though she was stubborn.

4. 当表示“推测”时，只能使用can't be done, 不能用be able to be done。道理就是，后者没有“推测”的用法。

非常遗憾的是，葛传槼教授只是给了一个“物”作主语的例句：The mistake is able to be corrected. 而Swan 先生给的例句虽然是“人”作主语，但对于He can't be understood. 这个句子，是表“推测”还是表“能力”，属于“模棱两可”，所以，《牛津英语用法指南》的解释，也没有完全否认He is not able to be understood.

【说明】《牛津英语用法指南》是一部外国人写的英语惯用法，其权威性不容置疑。但是，在阐述某个问题时，可能会出现“顾此失彼”的现象，我们可以从这本书上找出不少这方面的案例。这就像中国人写的汉语语法一样，也是存在不少争议及缺陷。关于英语的某个现象，只要现实语言中有大量足够的例证，就可以证明某个问题的是与非。