Two Ways to Join Sentences:  Subordination and Coordination

Clauses:

A clause consists of a subject and a complete verb, along with its modifiers.  Clauses can either be independent (meaning that it can stand alone as a sentence) or dependent or subordinate (meaning that the clauses must be joined to another sentence).

Independent: 

My dog can catch a Frisbee in her teeth.
Dependent (fragment, wrong): Because my dog can catch a Frisbee in her teeth.

Subordination:

You subordinate one sentence to another when you join two of them together in order to emphasize the ideas in one sentence or when you want to show a connection between two sentences.  Whenever there is a relationship between two sentences that can be expressed with a subordinator, it usually best to do it.  Instead of talking about two sentences joined together, it is more customary to refer to one sentence consisting of two clauses.

 Here is a list of common subordinators:

Time:

when, whenever, after, as, before, once, since, till, until, now, that, while, as long as, as soon as.

Concession:

though, although, even though, if, while.

Contingency:

if, once

Condition:

if, in case, as long as, unless provided that.

Reason:

because, since, as long as.

Result:

so, so that.

Comparison:

as, just as, as if.

Contrast:

while, whereas.

Subordinate clauses at the beginnings of the sentences are always set off by commas.  When the subordinate clause is second, there is usually no comma, except in front of  even though and although.

Coordination:

 You can join two independent clauses with (1) a conjunction, (2), a semicolon,  (3) a semicolon and a conjunctive adverb (adverbial conjunction), or (4) a colon.

1.

Coordinating conjunctions (coordinators):  and, or but yet, nor, for.  (Use the acronym BOYSFAN to remember them:  But, Or, Yet, So For, And, Nor.)

Our family has a cat, and we also have two dogs.
Sophie is a German Shepherd, but Grice is a mixed-breed.
Sophie is not much of a lady, nor is Grice a gentleman.

2.

The Semicolon:  Use to join two sentences when the idea in each is related and when you want to give equal emphasis to both ideas:

Sophie is an obedient and responsive dog outside; Grice is aggressive and hard to control.
The dogs general sleep when we do; they have completely adapted to our schedules.

3.

Conjunctive Adverbs and a semicolon:  use to express a relationship between two sentences.  Adverbs can be moved around within the sentence, but a conjunction cannot.  The semicolon is really what is joining the sentence, but the conjunctive adverb can express the relationship between the two sentences. 

With a conjunctive adverb:

Sophie is a German Shepherd; however, Grice is a mixed-breed.
Sophie is a German Shepherd; Grice is a mixed-breed, however.
Sophie is a German Shepherd; Grice, however, is a mixed-breed.
With a conjunction: Sophie is a German Shepherd, but Grice is a mixed-breed.
Wrong: Sophie is a German Shepherd; Grice is a mixed-breed, but.
Wrong: Sophie is a German Shepherd; Grice, but, is a mixed-breed.

Here is a list of many conjunctive adverbs:

Result:

therefore, consequently, as a result, of course

Concession:

nevertheless, yet, at any rate, still after all, of course

Apposition:

for example, for instance, that is namely, in other words

Addition:

moreover, furthermore, also, in addition, likewise, further

Time:

meanwhile, in the meantime

Contrast:

however, instead, on the contrary, on the other hand, in contrast, rather

Summary:

thus, in conclusion, then

Reinforcement:

further, in particular, indeed, above, in fact

These special adverbs are almost always separated from the other parts of the sentence by commas.  When they occur in the middle of the sentence, they should have commas on both sides.  When they are at the beginning or end of a clause (sentence used as part of another sentence), they are followed or preceded by a comma.

4.

Colons are similar to semicolons, except that with a colon, the second clause further explains or amplifies the first.  It’s as if you were writing the same thing twice, but in different words.

My dog has a lot of anxiety:  she appears to worry a good deal about me.