What is a Sentence?

A sentence, at its core, is merely a collection of words that gives meaning.  In fact, different languages have different standards for what is required in a sentence and how they're structured.  English only requires two things: a subject and a verb.

He runs.

In fact, English is called a Subject-Verb language, but this is only part of the truth.  Everything that is not the subject is also known as the predicate. 

Any sentence with a single independent clause (a subject-predicate construction) is called a simple sentence.

The rat gave fleas to his wife.

The rat is the subject of this sentence.  Gave is the verb.  The fleas are what are known as the direct object.  Finally, the wife is what as known as either the indirect object or the object of the preposition. 

A lot of the time, young writers, in some vain attempt to sound academic, like to squeeze as much information into sentences as they can.  Usually the result looks like thus:

The rat gave fleas to his wife she filed for divorce.

When you have two independent clauses jammed together, you create a compound sentence.  This, however, is not a compound sentence.  This is a run on sentence, because the two clauses are joined together with nothing but spit and a prayer.  So this is usually the solution students attempt:

The rat gave fleas to his wife, she filed for divorce.

This is no solution at all.  This creates another, more insidious error called a comma splice.  A comma splice is a run on with jewelry.  It's got bling.  That little dangling earring of a punctuation is certainly pretty, and looks nice between those two burly independent clauses, but it's about as effective at holding them together as a paperclip is at holding 2x4s steady.  It just won't do it.

Want to know the solution?

Nail that sucker in!

The rat gave fleas to his wife; she filed for divorce. 

By nailing in the comma with a period, and thus creating a semicolon, you have created a punctuation that is sturdy enough to hold two independent clauses together.  Really, the semicolon is the bastard lovechild of the period and the comma, taking up where the duties of both are not up to the task.  Coming to the end of an independent clause?  Use a period.  But you don't want to end the idea?  Use a comma.  But that's grammatically incorrect! Use a period.  But the next idea is linked to the first.  Use a comma.  But I should end the sentence!  Period.  But I don't want to end the sentence! Comma.

Aw, forget it.  Let's just use both.  Semicolon.

Of course, if you don't want to nail in your independent clauses, you can always mortar them together.

The rat gave fleas to his wife, and she filed for divorce.

Conjunctions are the glues that hold independent clauses together, although the different glues have different effects.  And, but, or, and for are the most common coordinating conjunctions, and they each create different meanings in a sentence.  Be sure to use the proper glue for each set of independent clauses.

Of course, if you're really sold on using that comma -- I mean, it certainly is pretty! -- then you can always revise the sentence so that it is no longer two big, inflexible independent clauses into a more flexible and lightweight structure.  Turn one of the clauses into a dependent clause (sometimes known as a subordinate clause).  A dependent clause is a clause that cannot stand alone as a sentence the same way independent ones can.

After the rat gave fleas to his wife  is a fragment.  A dependent clause is quite literally dependent on more information, or more specifically, an independent clause.  This is also why your elementary school teachers told you, rather erroneously, that you cannot and must not ever, ever, ever, ever begin a sentence with because; however the word because only creates a dependent clause, so were you to finish the sentence off with an independent clause, it would be correct.  In fact, let's change that after in the example into because.

Because the rat gave fleas to his wife, she filed for divorce. 

Because of the flexibility of this structure, the comma, as I said earlier, is well up to the task of holding this together.  This more efficient and elegant structure is known as a complex sentence.

We'll get to paragraphs later, but suffice it to say that the best strategy when writing is to change up structures based on need.  A paragraph of all simple sentences or all compound sentences or all complex sentences becomes droningly rhythmic.

I have one last word on sentences, however: active voice.  Okay, those are two words.  While many teachers ask for the active voice, I will, in fact, demand it.  Remember my three goals of writing: Clarity, Concision, and then Elegance.  The active voice lends itself to all three.  Here is all you need to know about the difference between the active and passive:

In the active voice, the subject verbs, or the subject verbs the object.

In the passive voice, the subject gets verbed.

Be kind to your subjects; don't let them get verbed.

~Prof. Peter