CS Comma splice
Don’t join two full sentences with a mere comma. If they are extremely short, it might be OK.
yes: Mary shot John with her pistol; he died a few weeks later.
yes: Mary shot John with her pistol. He died a few weeks later.
yes: Mary shot John with her pistol, and he died a few weeks later.
no: Mary shot John with her pistol, he died a few weeks later.

DP Double predicate
When you have two verbs with only one subject, don’t put a comma in between the verbs.
yes: John ate and drank.
yes: John ate, and he drank.
no: John ate, and drank

frag Sentence fragment
You have used a period or a semi-colon to set off something that isn’t really a full sentence.
no: Although she didn’t work for the KGB.
no: Their greatest contribution, the invention of the flying buttress.

MM Mixed-up modifier
You have used a modifying phrase inaccurately. Most likely, you have broken the rule that states that a participle phrase at the beginning of a sentence modifies the subject of that sentence.
no: Flying over Paris, the Eiffel Tower is very noticeable.
no: I saw my mother’s house flying over Washington.
yes: Flying over Washington, I saw my mother’s house.

PE Parenthetical expression
A parenthetical expression should have a comma before and after; if it is very short, it can do without any commas. To use only one comma is wrong.
yes: Mary, who was a spy, carried a pistol.
no: Mary, who was a spy carried a pistol.
no: Mary who was a spy, carried a pistol.

RO Run-on sentence
If two complete sentences are joined with a conjunction, you need a comma before the conjunction.
yes: Jesus ate, but he drank as well.
no: Jesus ate but he drank as well.

RW Rewrite
Something is wrong with your sentence; it needs to be redone.

sp. Spelling
A word is spelled wrong. It might be something tricky—e.g., "phenomena" is the plural of "phenomenon," but many people don’t know this.

WW Wrong word
Better look the word or phrase up in a dictionary. (Read the dictionary entry carefully!)