Punctuation with conjunctions:

You do not need to add punctuation if the clauses are short.  If they're long, however, and especially if you use "but," it is a good idea to add a comma before the conjunction:

     I went to the store during my lunch hour at noon, and I bought a loaf of bread.
     I did not want to go to the store on a busy Saturday afternoon, but I needed to buy bread.

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Punctuation

Punctuation is one of the most complex and difficult areas of grammar.  However, there are certain rules and tricks that can make things easier.  Keep in mind that the rules of punctuation are designed to make writing logical and easy to understand.

Semi-colon:

The mark that students (and others) have a great deal of trouble with is the semi-colon.  The semi-colon's main purpose is to connect two ideas that are closely related to each other, and in fact the second one usually offers some sort of explanation of the first.  In grammatical terms, here is the rule:
Use a semi-colon only between two main clauses:

     I had to go to the store; I needed to buy bread.
     This is not the first time he got into trouble; he had problems last year, too.

Here's a simple trick: since a semi-colon comes between two main clauses, each of which could form a sentence all on its own, you should be able to replace a semi-colon with a period.  If you can't substitute a period for your semi-colon, you're using it incorrectly and should find another punctuation mark.

(There is one other use of semi-colons, but it's rare and you may never have need of it.  The semi-colon is used to separate items in a list with internal punctuation:

     He invited Jan, a doctor; Anne, a lawyer; and Bill, a teacher.

This way, the reader knows that "a doctor" refers to Jan, for example, and not to a different person altogether.)

Colon:

Use a colon to introduce a list, even if it is a list of only one item.  The colon is especially useful after a main clause that includes a number or a classification.  What follows the colon can be a word, phrase, or clause:

     The book deals with two important issues: health care and the environment.
     I returned to school for one main reason: interest. (word)
                                                                   love of learning. (phrase)
                                                                   because I love to learn. (subordinate clause)
                                                                   I love to learn. (main clause)

Use a colon to introduce a quotation if your words don't flow smoothly into the quotation:

     Kent says that we cannot afford to wait: "We have to act now to save the planet."
     (But: Kent says that we "have to act now to save the planet.")

Comma:

The most used and abused punctuation mark is the comma. 
Use a comma before a conjunction (see above).
Use a comma to separate the items in a list.  If you use it in a list, add one before the final item in the list:

     lions, tigers, and bears

 Use commas to set off a parenthetical word or statement. 
Sometimes, you don't need or want to set it off; it depends on whether what you've added is essential to understanding the sentence.  If you use a clause that's necessary, it's called a "restrictive clause" and you should not use any commas.  If the clause is unnecessary, but just provides extra information, use commas to set it off.  The point is that in such cases use two commas or none:

     The team that gets the most points wins the game. ("that gets the most points" is a restrictive clause; it
          provides essential information)
     The team, which won the game last week, was given a parade today. ("which won the game last week"
          is a non-restrictive clause)
     Diane, the shortstop, will bat second.
     Martin returned and, exhausted, went straight to bed.

Use a comma after an initial adverb: that is, one that comes at the beginning of a clause:

     Unfortunately, I forgot to buy bread.   

Comma Splice (CS)

There is one thing that you cannot do with a comma: you must not use a comma alone to join two main clauses.  This error is known as a comma splice and is one of the worst grammatical faults:

     I went to the store to buy bread, I found that I did not have enough money.
     I went to the store to buy bread, unfortunately, I did not have enough money.

Revise the sentence by adding a conjunction or using a semi-colon:

     I went to the store to buy bread, but I found that I did not have enough money.
     I went to the store to buy bread; unfortunately, I did not have enough money.

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Conjunctions and Conjunctive Adverbs

There is a group of words known as conjunctive adverbs that cause a great deal of confusion.  The reason is that they seem to be conjunctions but they aren't.  Most adverbs end in "-ly" but conjunctive adverbs do not and so are easily mistaken for conjunctions.  "However," for example, seems like a fancier way of saying "but," while "moreover" and "furthermore" seem like fancier ways of saying "and."  It is important to remember that these are two different kinds of words and you shouldn't use conjunctive adverbs as conjunctions.
The most commonly used conjunctive adverbs are:

     furthermore, however, moreover, nevertheless, therefore, thus (sometimes)

The problem arises when the conjunctive adverb appears at the beginning of a clause.  It could appear anywhere, just like any other adverb, but when it appears at the beginning it seems to be acting as a conjunction.  Let's compare a couple of adverbs--first, we'll look at one that ends in "-ly" and thus looks like an adverb:

     Unfortunately, I did not have enough money.
     I did not, unfortunately, have enough money.
     I did not have enough money, unfortunately.

Now, let's substitute a conjunctive adverb:

     However, I did not have enough money.
     I did not, however, have enough money.
     I did not have enough money, however.

Because "however" seems to be like "but"--but with more syllables and therefore apparently more elegant and formal--students often use it as a conjunction:

     I went to the store to buy bread, however I did not have enough money.

The problem is that "however," not being a conjunction, does not join two main clauses together; instead, it is part of the second main clause.  As a result, two main clauses are being joined together by a comma alone, in the absence of a conjunction, and so you get a comma splice:

                 FIRST MAIN CLAUSE                                     SECOND MAIN CLAUSE
     [I went to the store to buy bread], [however I did not have enough money]

Note that "however," like other initial adverbs, must be followed by a comma (see above).  To join together two main clauses, use a semi-colon:

     I went to the store to buy bread; however, I did not have enough money.

Most commonly, students incorrectly use conjunctive adverbs in place of conjunctions.  Sometimes, however, they use conjunctions (especially subordinating conjunctions) where they should use conjunctive adverbs:

     Although, I found that I did not need to buy it after all.

The word should be "However" or "Nevertheless"--or even "Unfortunately"...

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