Yes, a writer needs to know what a sentence is.

I’m often struck by the way some writers seem to have no idea. To avoid this, make sure you understand what independent and dependent clauses are.

Once you understand independent and dependent clauses,  you will be able to write proper sentences and:

• avoid run-on sentences (e.g. comma splices)
• avoid sentence fragments

Independent Clauses

An independent clause is a complete phrase consisting of a verb and usually a subject, sometimes an object or predicate.

For example:

• “Walk.” (This is a complete sentence, so long as it is an imperative — that is, a command.)
• “I did.” (Another complete sentence, unlike this one.)
• “Obviously, this is also a complete sentence.”

An independent clause needs to be terminated with:

1. strong punctuation (i.e. a period, dash, colon, or semi-colon)
   OR
2. a comma and a coordinating word – but, and, because, etc.

For example:
“This is also a complete sentence, but now it has a second clause too.”

But this is an example of a run-on sentence:
“This is a also complete sentence, now it has a second independent clause.”

Specifically, this kind of run-on sentence is called a a comma splice — because it splices two independent phrases  together. Stronger punctuation than a comma is required here — a period, dash , or semi-colon. (Or a coordinating word like ‘but’ can be re-inserted.)

Summary: How to Avoid Run-On Sentences

It’s easy! Don’t put two independent clauses together in the same sentence without strong punctuation or a coordinating word and a comma.

Of course, the trick is to recognize independent clauses. This may be a little easier if we look at what makes a dependent clause, for contrast.

Dependent Clauses

A dependent clause is a phrase introduced with a coordinating word (a “subordinate conjunction”) that signals it is an incomplete thought.

These words include after, although, as, as far as, as if, as long as, as soon as, as though, because, before, even if, even though, every time, if, in order that, since, so, so that, than, though, unless, until, when, whenever, where, whereas, wherever, and while

A dependent clause cannot form a complete sentence.
E.g. “While this is an incomplete sentence.”

More Coordinating Words

Some coordinating words do not signal a dependent clause

• The conjunctions for, and, nor, but, or, yet, and so (FANBOYS)
  E.g. “Obviously, this is also a complete sentence, but now it has a second clause too.”
  This second clause is still an independent clause. It is equally correct to write:
  “Obviously, this is also a complete sentence. But now it has a second clause too.”Revered authors regularly begin sentences with conjunctions, in even the most formal writing, if you’re having doubts. Wikipedia lists examples from Shakespeare, Jefferson, Twain, Lincoln, Melville, the U.S Constitution, the  Atlantic, the Economist, the UK and Canadian Supreme Courts, etc.

• The same goes for comparative words like “Either… or,” “Not only… but,” “No sooner… than”
  E.g. “Either you accept that your grade school teacher was wrong, or you continue to  believe that it is incorrect to start a sentence with ‘and.’”

• Less controversially, this is also true of independent markers like also, consequently, furthermore, however, moreover, nevertheless, and therefore.

Sentence Fragment

An incomplete sentence can also occur if there’s a missing:

• Verb
  E.g. “A detailed neighbourhood in a city or intergalactic civilizations.”
• Subject (and it’s not an imperative or command)
  E.g. “Finding a place in a changing world.”

Summary: How to Avoid Sentence Fragments

• Is there no verb and no command? Is there no subject?
• Is it a dependent clause, as signalled by a word like “because”?

Then it’s a sentence fragment.