这个句子怎么理解，感觉是主语从句?

I wanted to see if it was possible for the exercises I loved doing, that I had seen transform healthy older people, to work in an Alzheimer』s unit. 但是 I had seen transform 这块有点看不懂

I wanted to see if it was possible for the exercises I loved doing, that I had seen transform healthy older people, to work in an Alzheimer』s unit. 但是

这句话主句结构是I wanted to see if it was possible for the exercises... to work in an Alzheimer』 unit. 其中，exercises有两个限制性定语从句，第一个I loved doing省略引导词that，第二个就不能省略了。你的疑惑是第二个定语从句（它不是主语从句），that I had seen transform healthy old people，that作had seen 的宾语，transform... 是动词不定式短语作宾语补足语。"我想知道我喜欢的、我见证过使老年人变得健康的这些运动用在阿尔兹海默症上能不能管用「。

其实不必纠结于that前面那个逗号，这里逗号用在两个定语从句中间，应该是作者（或其他natives）的习惯，用停顿来代替and。

这句话题主不是在「正牌」书刊报纸上看到的吧？很可能是某个 blog 上的：因为写得很随意，语法上并不严谨，不像是出自 100% 靠笔杆子吃饭的作家、记者、编辑之手，或至少是不曾经过编辑把关的。

题主如果还在被类似这样的长句困扰的话，其实目前还不适合看英美人写的博客，因为大多数的博客里，这类「半病句」太多了，对你提高英语水平没什么好处。同样花时间读英文的东西，不如多看一些起码经过编辑把关的文字。

这句话如果把它写得正规一些的话，应该是： I wanted to see if it was possible for the exercises, which I loved doing and which I had seen transform healthy older people, to work in an Alzheimers unit. 2 个 which 指的都是 the exercises。

相信金老师及其他几位答主已经为题主解惑了。我说说这俩定语从句。按题主在金老师的回答下的回复，我看了下原文。这是Psychology Today记者/编辑与英语教授、戏剧艺术博士Anne Basting之间的访谈录。题主发的这句话是Basting的回答部分。

按金老师说法改为并列定语从句确实更地道，但表现不出从句套从句的紧密逻辑关系。限制性定语从句中，一个先行词后接两个以上连环式（或称嵌入式）定语从句：N+定语从句1+定语从句2……，这时，定语从句2的先行词是「N+定语从句1」这个整体，也就是经过定语从句1限定过的N为先行词。

至于that前加了逗号，我的理解是表示停顿，以防误解：I wanted to see if it was possible for the exercises I loved doing that I had seen transform healthy older people, to work in an Alzheimer』s unit. 此时把I loved doing看作exercises的定语从句，然后将that看作指示代词，后面省略关系代词which，that作doing的宾语。凌乱不堪。而加了逗号就免去这些歧义。

所以，这个that前尽管有逗号，理解和翻译时依旧得按限制性定语从句处理。

说起Psychology Today，本文的记者和编辑Abigail Fagan的简历上这么写的：She has a bachelor』s degree from the University of Rochester where she majored in cognitive science and minored in English literature. 看来只能将限制性定从理解为非限了。

if 到句尾是宾从。

其中 it 是形式主语，实际主语是句末的不定式短语 to work in an Alzheimer』s unit。

而 that I had seen transform healthy older people 这部分按照意思和其本身的结构，应该是 that 引导的定从，与其前面的 I love doing 一样是修饰 exercises 的。不过我还是第一次看到两个限制性定从修饰同一个先行词时用逗号连接的。

1.I wanted to see if it was possible for the exercises to work in an Alzheimer』s unit. （句子主干，I wanted to see）

2."to see"后的"if"引导宾语从句。其中it指代to work in an Alzheimers unit，动词短语作主语。

3.(the exercises) I loved doing, (the exercises) that I had seen transform healthy older people, (定语从句）

4.I had seen transform healthy older people = I had seen (the exercies) transform healty older people.

我认为应该是定语从句。

大概的主干 I want to see if it was possible for the exercises to work in an Alzheimers unit. (我想知道这类练习/锻炼在这种针对阿尔兹海默症患者的特殊病房里是否有可能有用。)

我的感觉是 "I love doing" 和 "that I had seen transform....." 两个从句都是定语从句，修饰的是先行词 "the exercises"，第一个从句省略掉了关系代词(因为先行词做了直接宾语)，后一个句子似乎是因为没有直接和先行词相连，所以它的引导词(关系代词)that不可省略。

用到的结构是see sb./sth. do something (看到某人/某物做了某事)，所以第二个定语从句"that I had seen transform....."如果带入回原本的先行词exercises变回完整句子应该是"I had seen these (kind of) exercises transform healthy old people." (大概意思应该是"我曾经见到过这样的锻炼方法让老人们变得健康"? 我瞎翻译的hhh 不是很严谨)

重点是 I loved doing和that i had seen 都是说exercises的，所以是从that i had seen 这里断开。简化一下，exercises transform healthy older people to work in Alzheimers unit.