it was in 1949 ___ China was founded.知道是强调句要填that,但为什么填which和when不行,错在哪里?it was in 1949 which China was founded 语法哪里错

用that,是强调句。
首先，1949时间前有介词in，要是用when，则1949作状语，前面不需要用in，即it was 1949 when China was founded.（这是定语从句）
其次，你可以还原句子：China was founded in 1949.这里不缺when，是完整的句子——这就是强调句的特点，只把要强调的部分（主语、宾语、状语等）放在it is（was）that（who）后，其他成分位置顺序不变，也不缺失，如I saw Tom on the street yesterday.强调宾语：It was Tom who(that) I saw yesterday.强调状语：It was on the street that I saw Tom yesterday.强调主语：It was me who(that) saw Tom on the street yesterday.
it is（was）引导的 定语从句 在 强调状语 时都没有介词，如it was 1949 when China was founded.It was the party where I met her.It was the day when my cat died.
在此，只用记住一点：in+时间=when；in+地点=where.
回答完毕，望采纳

填‘that ’
是强调句，判断的依据是去掉It was ...that之后句意不变。
若要填which和when从句应该为定语从句。
举一个例子：
it was on sunday that we went to the park.
强调句
it was sunday when we went to the park.
定语从句
关键就在于前面的介词on
有on的话去掉it is ... that句子是完整的所以就是强调句，没有的话就是定语从句
所以本句也是类似。

因为是强调句，必须用THAT，这个是固定的(个别情况也可以用 WHO），句式规定死的，没办法

因为是强调句啊
呵呵，这问题你让大家怎么答给你啊，你自己都知道。

定语从句，which应该有指代的东西在前面，你看一下，它在这里能指代什么？什么都讲不通，所以不对，是强调句，固定名式

It was ……that 就表明这是一个强调句了，后面就一定要用that！这是常考的，没有为什么，英语就是这样，有时无法解释，是约定俗成的
it was in 1949 which China was founded 语法哪里错??反正就是错的！你问老外他们也不知道

强调句判断的依据就是去掉It was ...that之后句意不变,语法通顺.
It was in 1949 that China was founded.
In 1949 China was founded.
你就记住把it was和that去掉看句子语法对不对 完不完整就可以了.
如果填which和when从句应该是定语从句 修饰1949这个年份
It was 1949 when China was founded.
It was 1949 in which China was founded.
题目上的句子中in 1949这个介词短语在句子中做状语
用it is…that强调句就是强调这个状语
而把in去掉只剩1949的话整个句子不完整了

固定句式嘛，英语里的习惯表达
就像中文里经常说“饭吃了没”而极少用“你有没有吃饭”一样 语言的特点