When writing a compound sentence -- that is, a sentence composed of more than one independent clause -- the two ideas of equal importance known as clauses must be connected somehow so that the reader knows the two ideas are related to each other. Coordinating conjunctions serve to accomplish this task; there are seven coordinating conjunctions in the English language: for, and, nor, but, or, yet, and so.
A mnemonic device to remember these coordinating conjunctions comes from taking the first letter of each word to come up with the word FANBOYS. Each of these coordinating conjunctions accomplish the same task--connecting compound sentences--but each one of the coordinating conjunctions adds a different meaning to the sentence.
Coordinating conjunctions most often occur in the middle of a compound sentence, and more often than not, they are preceded by some sort of punctuation--most often a comma, but not exclusively. Here is an example of coordinating conjunctions in the middle of sentences:
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Tommy went to the store, and he made dinner when he returned.
In this case, Tommy does two distinct actions, both of equal importance. Therefore, the coordinating conjunction "and" indicates to the reader that the two clauses--"Tommy went to the store" and "he made dinner when he returned"--are of equal importance and are logically connected.
Here is another example:
Tommy went to the store, but he forgot to buy milk.
In this case, the word. :but" acts as a coordinating conjunction to indicate that the two independent clauses relate to each other; in this case, the second clause indicates a reliance on the first clause. "But" typically indicates some sort of exception to the first clause.
Another example that demonstrates the use of coordinating conjunctions:
Maria still wanted to dance with Tommy, yet she couldn't muster the courage to ask him.
In this case, the second clause is related directly to the first clause, but the coordinating conjunction--"yet"--indicates that there is more depth to the first clause. This depth is qualified by the second clause. Generally, the conjunction "yet" indicates two ideas that are logically connected but somehow contradictory.
One final example:
Tommy did not own any books, so he decided against putting up bookshelves.
In this case, the coordinating conjunction "so" indicates that the second clause is a consequence of the first. Tommy does not own any books, so he does not need bookshelves. The lack of bookshelves is a result of his lack of books.