English question: Determine which sentence type each of the following sentences belongs to (simple sentence, parallel sentence, compound sentence)

Both are compound sentences.

The first one is the subject clause, What I want to know serves as the subject in the sentence.

The second is a predicative clause, why I 've come to see you is used as a predicative in the sentence.

A simple sentence is a complete sentence with various necessary components; a parallel sentence is two sentences with complete and independent components combined into one sentence, connected by conjunctions such as and, but, etc.; in a compound sentence, except In addition to a simple sentence with a complete structure, there will also be independent adverbial clauses, etc.

Use interrogative words

as introductory words. The main clause is the main body of the sentence and can usually exist independently; the subordinate clause is a sentence component and cannot exist independently. A clause cannot stand alone as a sentence, but it also has a subject part and a predicate part, just like a sentence. The difference is that the clause must be introduced by a conjunction. According to the different functions of introductory clauses, they can be roughly divided into: subject clauses, predicative clauses, object clauses, attributive clauses, adverbial clauses, etc.