A complete sentence has a subject that is described or acted upon. This means that it requires a noun and verb that interact. In the example I am about to show you, the incorrect version is divided into two separate sentences; the second is the incomplete one, or a sentence fragment.
Example of incomplete sentence: The clock had frozen. Which was okay with him, since he hated the ticking noise.
Example of complete sentence: The clock had frozen, which was okay with him, since he hated the ticking noise.
To put it in very simple terms, a clause is an independent part of a sentence. It has its own subject or sub-subject that relates to the main subject. Separate clauses are most commonly separated by commas. Introductory clauses require periods, too. The only time that an introductory clause can go without a period is if the sentence consists of six or fewer words, which is what const