Question

If she could sew ,she would have made a shirt。 “If she could sew ”这句语法有没有错

Answer 1

这个句子存在的问题是If虚拟语气中主从句搭配混乱，主句表示同过去事实相反时，从句则和现在事实相反。
答案应为：If she could sew ,she would made a shirt。与现在事实相反的假设。她现在要是会缝纫的话，她就能做衣服了。
或者是If she had sewed ,she would have made a shirt。与过去事实相反，他要是缝纫的话，他现在就做好衣服了。
这个是If虚拟语气的用法，之所以要用虚拟语气，一般是因为这件事和事实是相反的，不可能的。具体用法是

当与过去事实相反时 主句用would/could/might/should+have done
从句用had done
例句：
If he had know your address yesterday,he would have telephoned you
如果昨天他知道你的地址，那他就打给你了。
If he had been there last night,it wouldn’thave happened.
如果他昨晚在那里，这事就不会发生。

与现在事实相反时 主句用would/could/might/should+do
从句一般过去时表示将来（be 动词都用were 哦）
例句：If I place were alike ,there would be little need for geographers.
If I were you,I should do that.如果我是你，我就那么做。

与将来事实相反 主句用would/could/might+do
从句用1were to do2 一般过去（be 动词都用were 哦）3should+do（三者取其一哦）
例句：
If anyone helped me,I would make him very rich.
如果有人把我放出来，我会使他富有。
If it should rain the crops would be saved.
如果有雨下，庄家就有可能救活。
If it were to snow tomorrow,they wouldn’t go for an outing.
如果明天下雪，他们就不外出游玩。

Answer 2

If she had been able to sew,

Answer 3

没错

追问

If she could sew 虚拟语气中不能用could吗

Answer 4

选b
if
的这种句子分三种。
if-conditional
i(1),
if-conditional
2
和
if
conditional
3
你这个句子现在是第二种，一般是不太可能完成的一个条件。比如说“if
i
had
1million
dollar,
i
would
travel
around
the
world.
但事实上是”我“是不太可能有这100万美元的，所以用第二种虚拟的if-conditional。中文里语法叫什么我不是很清楚，只知道英语的，请原谅下。
第二种的在if句里面用simple
past
（简单过去式），在主句里面用would+动词原型