Number seventeen on the list of the twenty most common grammar errors is the unnecessary comma with restrictive element.
I have taken a lot of German classes, and I can actually speak a bit of the language, which is exciting to me. In my last class, my instructor had us write a lot of essays, and the one item I got marked on a lot was my commas. My instructor told me that was a common problem among English speakers.
English writers are comma crazy.
In error number 5, no comma in non-restrictive element, we learned that a non-restrictive element was really a parenthetical and should be sent off by commas. Whereas a non-restrictive element is not necessary to the understanding of the sentence, a restrictive element or an essential element is vital to the meaning of the sentence.
I like the examples of restrictive elements on About.com’s grammar portion of their site, so I am reproducing them here with my own edits.
v A person who won’t read has no advantage over one who can’t read. –Mark Twain
§ A person has no advantage over one.
v A poet more than thirty years old is simply an overgrown child. –H.L. Mencken
§ A poet is simply an overgrown child.
v It is impossible to imagine Goethe or Beethoven being good at billiards or golf. –H.L. Mencken
§ It is impossible to imagine Goethe or Beethoven.
What happened to the sentences when I removed the restrictive elements?
Notice also the missing commas. And if you add the commas, the commas drastically change the meaning of the sentence.
It is impossible to imagine Goethe or Beethoven, being good at billiards or golf.
Adding the comma to the above sentence changes the meaning. The sentence now means: if you are good at billiards or golf, then it is impossible for you to imagine Goethe or Beethoven.