Common punctuation marks are periods, commas, question marks and exclamation points. Sentences that make a direct statement end in a period. Periods also indicate abbreviations. Rules call for not using an additional period if the sentence ends in one used for an abbreviation.
Commas indicate pauses, separate clauses and separate items in a series. Rules vary on their usage. Furthermore, varying opinions abound on using serial commas and the comma before the "and" at the end of the series. In general, commas separate independent clauses, introductory clauses and non-restrictive subordinate clauses. Interrogatory sentences end in question marks, while statements expressing excitement, commands or emphasis end in exclamation points.