改错：His mother is ill,he has to look after her at home.

这个句子不对!
你看中间是一个逗号 ,最后是一个句号.所以说明这一个长句子没有链接副词 或者连词
一下整个大句子是正确的：
His mother is ill,so he has to look after her at home.
His mother is ill and he has to look after her at home.
既然没有连接词 那么这句话是一句话 就是错误的!所以要写成独立主格的形式：
His mother being ill,he has to look after her at home.
= with his mother being ill,he has to look after her at home.=with His mother ill,he has to look after her at home.

如果是is的话，那就是两个句子，那中间应该加and，所以必须把一句改为状语，是因为他的妈妈生病，所以他才不得不留在家里照顾她，所以把前句改为原因状语，动词必须加ing引导原因状语从句

作状语部分，所以用being.如果用is那就是一个独立的句子，与下一句，应该有连词相连

首先这是一个原因从句当然要改成being了，原句中用的是","这就显然是一个从句。如要用is 原句应该为His mother is ill,and he has to look after her at home.