Commas are commonly used to: a) set off a modifying phrase or clause that beings a sentence; b) set off a transitional word, phrase, or clause that begins a sentence; c) set off a modifying element that ends or interrupts a sentence IF the modifier qualifies, contrasts, or makes an exception to the main clause.

Here's a source for more info on comma usage: http://www.wisc.edu/writetest/Handbook/Commas.html#definitions

In the sentences below, rewrite so that the transition or modifier comes at the beginning or end of the sentence, using commas correctly. Some are correct.

1. Bulgaria attacked SErvia and Greece in 1913 in a second war over boundaries.
2. Bulgaria was carved up as a result of the 1913 war by its former Balkan allies and Turkey.
3. The assassination of the Archduke Ferdinand of Austria the following year brought on the First World War.
4. A sprawling new nation, Yugoslavia, was formed after the collapse of the Austro-Hungarian Empire.
5. The aspirations of Croats and other minorities in Yugoslavia were suppressed under the tenuous domination of the Serbs.
6. The collapse of the Ottoman Empire at the same time left many ethnic Turks subject to their longtime foes the Bulgarians.
7. A million Armenians were slaughtered at the same time as a result of attempts at forging a new Turkish state in Anatolia.
8. Undermined by corrupt and meddling monarchs and by ethnic passions, parliamentary governments of southeastern Europe rose and fell.

Essential or restrictive modifiers don't use commas. Nonessential or nonrestrictive modifiers need commas.

In the sentences below, combine the pairs of sentences and use commas to set off nonessential modifiers or no commas around essential modifiers.

1. Film by its very natujre is better able than prose to present the event in all its intensity. Even the most sober historians are willing to admit that fact.
2. In some instances the film has turned out to be more historically accurate than the original historical account. Vivien Leigh's portrayal of a spirited Scarlett O'Hara is now considered to be a fairly accurate interpretation of the not-so-helpless Southern belle.
3. Of course there have been plenty of instances of mistakes in historical representation. A film might carefully reproduce the cultural material of an era but skew the facts of the event.
4. Viewers are more comfortable if the film ratifies their personal biases. Hollywood history films tend to reflect the biases of their viewers, especially in political matters.
5. Bonnie and Clyde transformed a vapid Bonnie Parker into an aggressive moll. Anne of the Thousand Days transformed an ambitious and strong-willed Anne Boleyn into a lovesick, awestruck girl.

1. Despite the fact that these people are so punctilious about the care of the mouth, this rite involves, a practice which strikes the uninitiated stranger as revolting.
2. It was reported to me that the ritual consists of inserting a small bundle of hod hairs into the mouth, along with certain magical powders, and then moving the bundle in a highly formalized series of gestures.
3. In addition to the private mouth-rite the people seek out a holy-mouth-man once, or twice a year.
4. These practitioners have an impressive set of paraphernalia, consisting of a variety of, augers, awls, probes, and prods.
5. The use of htese objects in the exorcism of the evils of the mouth involves, almost unbelievable ritual torture of the client.
6. The holy-mouth-man opens the client's mouth and using the above-mentioned tools enlarges any holes which decay may have created in the teeth.
7. Magical materials are put into, these holes.
8. If there are no naturally occuring holes in the teeth, large sections of one or more teeth are gouged out so that the supernatural substance, can be applied.
9. In the client's view, the purpose of these ministations is to arrest decay, and to draw friends.
10. The extremely sacred and traditional charactger of the rite is evident in the fact that the natives return to the holy-mouth-men year after year, despite the fact that their teeth continue to decay.