the cat likes fish ——is known to us

1.楼主应该是忘记了在fish后面加上一个逗号.
正确的写法和答案是：The cat likes fish,(as) is known to us.
2.
1）非限制性定语从句中,关系代词as代替整个主句the cat likes fish,做该定语从句的主语(从句).
即完整的定语从句为：(That the cat likes fish) is known to us.
猫喜欢鱼（这种情况）是我们所知道的.
2）整个句子可以翻译成：就像/正如我们所知道的那样,猫喜欢鱼.
2）which虽然也有上述用法,但是关系代词做主语,而其后是be动词时,只能用as,而且which不可以翻译成as所具有的“正如/就像...一样”的含义.

which
引导定语丛句，在这里Which代指前面的一句话（这个猫喜欢鱼）
如果是做一个单词的定语通常会填That.

as it