Commas and Starter Elements

It is very common for a sentence to begin with an introductory word, phrase or clause. You need to use a comma to separate the starter element from the main clause that follows. Here are some examples of starter words and phrases to watch out for:

• Starter clauses often begin with when, while, after, since, because, although, as, if, etc. You can learn more about subordinating conjunctions here.
  
  After the Trojans brought the wooden horse into the city, the Greek soldiers inside the horse waited until nightfall to begin their attack.
  Because they caught the Trojans by surprise, the Greeks were able to capture the city in a single night. If the Trojans had been more cautious, they might have been able to save themselves and their city.
  
  
• Starter phrases include participial phrases, infinitive phrases, absolute phrases, nonessential appositive phrases, and long prepositional phrases (especially prepositional phrases that are four words or longer).
  
  Grabbing hold of the sacred Palladium, Cassandra prayed to the goddess to save her. The Greeks having overrun the city, there was nothing that the Trojans could do. During that single night of terror, the Greeks won a war that had lasted for many years.

• Starter words usually provide a kind of transition from the previous sentence. Common starter words include however, nevertheless, yes, etc.
  
  The Trojans had put their trust in the gods and in their own strength. Nevertheless, they were defeated by the Greeks. The Greeks finally won the war. Yes, it was a great victory, but it came at a terrible price.
  

For more information, see Purdue's OWL.

---

For more about the Trojan Horse and the fall of Troy, see Wikipedia.

(image source)