Clause is built by two sentences. The first sentence is called "Sub Clause" because this sentence is added or placed a conjunction at the beginning (When the sentence is added by a conjunction, we don't call it "sentence" anymore...). and second, Main clause. Main Clause is a real sentence because it contains, at least, one subject and one verb as a sentence builder.
To read more about clauses, please click links below!
1. Noun Clause
2. Adjective Clause (Adjective Clause (2))
3. Adverb Clause
Have a nice day!