Noun clause:
the main job of four kinds of sentence elements: subject, object, predicative, appositive.
Noun clause can do three sentences: declarative, general questions, special questions.
Summarized in one sentence: a noun clause is in three sentences to each function as a component of another four kinds of sentences sentence.
Declarative sentence:
that + declarative, object clause of that can be omitted, subject, predicative, appositive clause can not be omitted.
General questions Q:
First Inversion general question into a declarative sentence word order in a sentence plus wether or if (if only in the object clause).