Once you have written a sentence, do not be tempted to put a comma and write another sentence. This is the most common mistake involving commas. It is called a run-on error.
(Before We Start with Run-On Error) What Is a Sentence?A sentence is a grammatically complete series of words.
A sentence must have a subject and a verb, even if one or the other is implied, and it must express a complete concept. A sentence begins with a capital letter and normally ends with a full stop (period ).
(NB: A sentence can also end in a question mark (?) or an exclamation mark (!). If we're being really pedantic, it could also end in a speech mark, but it definitely can't end in a comma.)
What Is a Run-on Error?Once you have formed a sentence (i.e., expressed a complete concept), you must put a full stop / period or another valid ending (like ! or ?) and end the sentence. Do not insert a comma and continue writing. This is a very common mistake. It is known as a run-on error, a run-on comma, or a run-on sentence. For example:
Your Idea Could Consist of Several SentencesWriters often feel that a comma is more appropriate than a full stop / period because their sentences are so closely related. In other words, they sense that a full stop / period is too abrupt because they haven't finished expressing their idea.
Remember, a sentence is a grammatically complete series of words. Often, it will take several sentences to complete your idea. Look at the examples below:
EXTENDING A SENTENCE WITH A SEMICOLON, A DASH, OR THREE DOTS
Run-on errors occur because writers feel that their ideas need to be crammed into single sentences. They don't. Occasionally, however, it may be appropriate to use a dash, a semicolon, or three dots to extend a sentence.
Read more about extending sentence with a dash, a semicolon, or three dots.
What are conjunctions?
Commas before conjunctions
Commas after a sentence introductions
Commas after a transitional phrase
Commas after interjections (yes, no, indeed)
Commas for parenthesis
Commas in lists
Commas with a long subject
Commas with numbers
Commas with quotation (speech) marks
Commas with the vocative case
Commas with Dear, Hello, and Hi
Semicolons to extend a sentence
Extend a Sentence (dashes, semicolons and three dots)