请问为什么这两句话，1句是名词性从句（noun clause）,而一句是定语从句（relative clause）?

1。 The party [which Pat said that Chris was going to this weekend ] is at Sandy's
1）先行词是 The party
2）关系代词 which 作 was going to 的宾语
3）定语从句分析：
a）主句：Pat said that...
b）宾语从句 which Chris was going to .

2。 The man [sitting at the desk] is the clerk.
1）主体结构：The man is the clerk.
2）sitting at the desk 是现在分词短语作后置定语，也可以扩展为定语从句：who are sitting at the desk,也就是说分词短语作定语是由定语从句省略关系代词和助动词后转变而来的，如：
This is the village which was occupied by Japanese invaders during the war.
—— This is the village occupied by Japanese invaders during the war.

3。 The fact [that he didn't attend] has everybody puzzled.
1）that he didn't attend 是同位语从句，属于noun clause，用来进一步说明一个抽象名词的具体含义。
2）判断定语从句和同位语从句的要点是：定语从句的引导词 that 在从句中充当语法成分（主语、宾语或表语），而同位语从句中的引导词 that 只是个语法符号，在从句中不充当任何成分，如：
The fact that he told me just now has everybody puzzled.
that 在从句中作谓语动词 told 的宾语，因此是定语从句。