定语从句里的that后面是可以跟is的。
一、定语从句的定义。
用作定语的从句叫做定语从句(attributive clause)。定语从句通常皆置于它所修饰的名词(或代词)之后，这种名词(或代词)叫做
先行词(antecedent)。引导
宾语从句的关联词为
关系代词和关系副词。关系代词在定语从句中可用作主语、宾语、定语等;关系副词在定语从句中只用作状语。如：
The student who answered the question was John. 回答问题的那个学生是约翰。(who answered the question是关系代词who引导的定语从句，用以修饰who先行词student，who在从句中用作主语)
I know the reason why he was so angry. 我知道他这么生气的原由。(why he was so angry是关系副词引导的定语从句，用以修饰why的先行词reason，why在从句中用作原因状语)
定语从句一般紧跟其先行词之后。如：
The room which served for studio was bare and dusty. 这个用作工作室的房间空荡荡的，布满灰尘。(关系代词which引导的定语从句紧跟其先行词room之后)
有时亦可与先行词分离。如：
A new master will come tomorrow who will teach you German. 明天要来一位新教师教你们德语了。(关系代词who引导的定语从句与其先行词master分离)
二、关系代词的用法。
用作关联词的关系代词有who，whom，whose，that，which等。who，whom，whose指人，who是
主格，在从句中用作主语(在非正式英语中亦可用作宾语);whom是
宾格，在从句中用作宾语;whose是
属格，在从句中用作定语(有时亦可指物)。如：
The man who was here yesterday is a painter. 昨天在这里的那个人是位画家。(主格关系代词who在从句中用作主语)
The man who I saw is called Smith. 我见到的那个人名叫史密斯。(在非正式英语中who 代替了whom，亦可省去不用)
I know the man whom you mean. 我认识你指的那个人。(宾语关系代词whom在从句中用作宾语)
A child whose parents are dead is called an orphan. 失去父母的孩子叫做孤儿。(属格关系代词whose在从句中用作定语，指人)
I’d like a room whose window looks out over the sea. 我想要一个窗户面临大海的房间。(属格关系代词whose在从句中用作定语，指room，可代之以of which，但后者较为正式)