What he said is true. 能改成That he said is true.嗎? 意思一樣? 上述What he said是名詞子句? 但he said不是不完整的句子?不完整的句子不是不能當名詞子句? 還是he said是完整的句子?

What he said is true.
= The thing that he said is true.
兩句都是complex sentence。
在第一句裡的what是「複合關係詞」。複合關係詞＝先行詞＋關係詞。
因此複合關係詞所引導的子句之前無需先行詞。用複合關係詞帶出來的子句是名詞子句，可以當作主句的主詞。在主句裡what兼作意義上的主詞，在附屬子句中為動詞的受詞。你不能將附屬子句裡的what拆出來，因為它是子句不可缺少的一部分。
第二句裡的that是「關係代名詞」，前面的名詞thing是它的先行詞，也是主句的主詞。 that he said是形容詞子句，當作「修飾語」，修飾先行詞。因為不是名詞子句，不能當作主詞。寫成That he said is true.是錯誤的，因為關係代名詞that的前面「沒有」先行詞。

"That he had lied to us" was unbelievable.
＝It was unbelievable "that he had lied to us."這兩句裡的that是「連接詞」。
後者的名詞子句是「延遲的主詞」，前面用了虛主詞it。

he said 是不是完整的句子?
No, it is not a complete sentence as "say" is a transitive verb (can be intransitive only in negative sentence) requiring to take on object..

(1) What he said is true.
"what he said" is called nominal relative clause, functioning as a noun clause, which can be the subject, subject complement and object of the sentence. "what" is a pronoun (= the thing that), which acts as the object of "said".

(2) That he said is true.
"that he said" is a typical noun clause, acting as the subject of the sentence. "that" is a conjunction here introducing the noun clause.
(cleft sentence) = It is true that he said.
As "he said" is incomplete, the noun clause as well as the whole sentence is wrong.

(correct sentences - It is true that he said yes. That he said yes is true.)

What通常用在問問題的句子 What he said is true?
That he said is true和what he said is true 意思不同
That he said is true這句是不完整的句子
如果想回答他的話說對的就應該用The things he said is true.