Although a small mark of punctuation, the comma is the most often used and the most often mis used mark of punctuation. The comma ordinarily indicates a pause and a variation in voice pitch. As a hint, you can often decide on the placement of a comma by reading aloud the sentence. However, as with many rules in English, this does not always help decide or assure placement. Commas are generally used to set off transitional expressions such as however, therefore, moreover, for example , and in other words .
Transitional expressions connect sentences or parts of sentences.
When a transitional expression appears between independent clauses in a compound sentence, the sentences are connected by a semicolon and a comma usually follows the transitional expression. As a matter of fact, the student was not present for the lecture that day. Using a semicolon eliminates the need for any comma. The movie began and I was entertained. Commas should be used to set off nonrestrictive clauses and phrases.