有關英文句子的疑問?

what是「帶先行詞的關係代名詞」，原句是用一個「名詞子句」在當主要動詞understand的受詞，而what在名詞子句裡兼動詞says的「受詞」。名詞子句的主詞是teacher。

I can't understand "what the teacher says in the class."（用名詞子句當動詞understand的受詞）
= I can't understand the things "that the teacher is talking about in the class."(用名詞things當受詞，然後用形容詞子句當其修飾語）

你加上一個is後，名詞子句的結構就遭到破壞，成了兩個動詞（is + says)，語法錯誤了。

I don't know what his name is.
看看「間接問句」的結構（當作動詞受詞的名詞子句），子句的主詞是his name，動詞要放句尾。

I can't understand what the teacher "says" in the class

為什麼不能用---if there are 2 verbs in a n clause---.I can't understand what "is" the teacher "says" in the class?!-----=(is+says)
To analysis a sentence with a noun clause with a verb:-eg:-
I-------------------------s
can't understand----main v
what the teacher says----If(osv),then To Antecedent Pattern with the,relative pronoun "what"--,then n.clause--------o
then svo=subj says what
=svo=the teacher says what--on rearranging
=eg:-Lion
n.clause=1 v only in analysing;
what is the teacher says------no 2 v in a sentence-clause=no n.cl. has 2 v=(says+is)
=svo
=the teacher is what--on rearranging
=eg:-lioness cat