Every sentence must be a complete sentence, with an independent clause consisting of both a subject (noun) and a predicate (verb).
Sentence-level errors will frustrate your readers, forcing them to stop to retrace their steps in order to figure out what you really meant to say. Notice the effect of the following error on your reading process:
The Aztecs conquered many peoples and established a tributary system, under Aztec power the subordinate villages paid tribute in the form of corn and other valuable items.
After reading the initial independent clause—the words leading up to the comma—you might naturally have expected that under would have begun a prepositional phrase modifying the word system. The sentence would have been fine, if it had proceeded as follows:
The Aztecs conquered many peoples and established a tributary system, under which subordinate villages paid tribute in the form of corn and other valuable items.
Instead, the original sentence violates basic rules of punctuation and syntax by splicing together two independent clauses with a comma, thus creating a RUN-ON SENTENCE. The original sentence above, then, would stymie careful readers, requiring them to reread the sentence while mentally correcting the error.
Our rules about punctuating sentences exist for a very good reason: the human mind does not easily cope with syntactical indeterminacy. The only way to makes sense of such a sentence is to go back to the beginning and start over.
Below are a few more examples of common sentence-level errors.
Wrong: Although the United States entered the Great War in 1917.
Right: Although the United States entered the Great War in 1917, the conflict actually began in 1914.
Comment: Notice that the wrong version would be a complete sentence without although. With although at the beginning, however, the clause is no longer independent—it is a FRAGMENT.
Wrong: Abraham Lincoln, writing a great speech, called the Gettysburg address.
Right: Abraham Lincoln wrote a great speech called the Gettysburg Address.
Comment: This error also creates a FRAGMENT. Notice that there are verb forms in the wrong version (writing and called), but they do not function as predicates.
Wrong: The Populist Party grew rapidly during the early 1890s, it lost its strength after backing the Democratic presidential candidate in 1896.
Right: The Populist Party grew rapidly during the early 1890s, but it lost its strength after backing the Democratic presidential candidate in 1896.
Comment: This serious problem is known as a COMMA SPLICE. The two parts of the sentences are both independent clauses, and they must either be joined by a comma and a conjunction (but, in the right version) or separated by a period or semicolon.
Comment: Please note that however is not a conjunction. If you use however in place of but, as in the first example below, you create a COMMA SPLICE.
Wrong: I enjoy visiting the zoo, however I am afraid to set foot in the reptile house.
Right: I enjoy visiting the zoo, but I am afraid to set foot in the reptile house.
Right: I enjoy visiting the zoo; I do not, however, set foot in the reptile house.
For more help with sentence errors, see sections 14 and 15 of the Pocket Style Manual, which includes multiple suggestions for identifying and repairing fragments and run-ons.
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