There are many, many rules governing how writers may use commas. Paranoid from red marks on their papers, some writers end up using commas wherever possible to avoid getting marked for mistakes. You should resist this urge! Unnecessary commas can be just as distracting as missing ones.
Conjunciones de coordinación
Most unnecessary commas come from misapplying comma rules. For instance, we discussed using coordinating conjunctions like and, but, or so, in our discussion of comma splices. Just because you need to use commas with coordinating conjunctions sometimes doesn’t mean you’ll use them all the time.
In this example, you don’t need a comma before the but because you aren’t combining two independent clauses. The second half of the sentence is dependent upon the first half. Here is a corrected version of the sentence:
Correlative Conjunctions
Another place writers might be tempted to insert unnecessary commas is when using correlative conjunctions. There are word combinations like either/or and both/and that connect a sentence.
Commas aren’t necessary when combining sentence elements with correlative conjunctions.
The exception to this rule is the correlative conjunction if/then, which always requires a comma.
Non-Essential Information
Have you come across a sentence like this?
In the above sentence, commas are used correctly to separate non-essential (but colorful) information from the rest of the sentence. Having seen this construction, some writers might end up using commas unnecessarily around information that is essential to the sentence.
En este ejemplo, para mi comida no es una información extra que deba separarse con comas. Pero, usted sí necesita una coma antes de "para " para separar las dos cláusulas independientes. Aquí tiene una versión corregida de la frase:
For more information on these comma rules, be sure to check out the Comma section in the Punctuation area of Grammar Essentials.