1. Mr. Eble CP2 Senior English
Common Error #4: Unnecessary Commas with a Restrictive / Essential Element
Mr. Eble
CP2 Senior English

2. A Restrictive Element is…
one that is essential to the basic meaning of the sentence—it restricts the subject of the sentence to a specific meaning. Removing it would remove information vital to the idea being communicated in the sentence.
EXAMPLE: Mr. Eble attacked the man who yelled at his daugter.
Many people include commas before / after these elements…

3. How to Check for Essential Elements…
Check any words or phrases in your draft set off with commas to make sure that the element set off is not a restrictive element, one essential to the basic meaning of the sentence.
Such essential words or phrases are not set off from the rest of the sentence with a comma or commas. 
Example: Moeller, which won its first four games, will play St. X tonight.

4. Restrictive / Essential Elements
Nonrestrictive / Nonessential Elements
Necessary to retain the meaning of the sentence.
Covering it up changes the message conveyed
One does not need to place commas around it
That
Acts as more of a side note rather than essential information.
If omitted, it does not affect the main clause of the sentence.
One must place commas around it.
Which