Full Stops

The full stop tells us that the grouped words we've just read are one logical entity or one organic whole. Reading aloud, when we reach a full stop we allow the voice to drop slightly, which tells listeners that we've completed the thoughts in that sentence.

Usually there will have been a couple of nouns (or pronouns) and a verb in there:

The player (noun) hit (verb) the ball (noun). Full stop - we get the picture.

The trouble with my students is that, when they want to say more, they add more without considering the structure. For example:

The player hit the ball, it flew over the stands.

ARGH!

Joining sentences

The words in that example form two sentences:

The player hit the ball. It flew over the stands.

If you want to make the two sentences into one, then use a word such as AND, BUT, WHILE, ALTHOUGH, BECAUSE, AS, THAT, WHICH. You can have a lot of fun discovering new meanings and subtle implications behind simple actions by playing with these.

The player hit the ball AND it flew over the stands.
The player hit the ball AS it flew over the stands.
The player hit the ball BUT it flew over the stands.
The player hit the ball WHILE it flew over the stands.
The player hit the ball ALTHOUGH it flew over the stands.
The player hit the ball BECAUSE it flew over the stands.

You'll find interesting effects can come from adding or omitting commas with the word "which":

The player hit the ball, WHICH flew over the stands.
The player hit the ball and, as a result, it flew over the stands.

The player hit the ball WHICH flew over the stands.
The player just hit that ball, but there might have been others. It might well have been flying over the stands before he hit it. If you meant this, it would help to change the verb from past to past continuous, or pluperfect:
The player hit the ball WHICH WAS FLYING over the stands. (Past continuous tense, something going on happening.)
The player hit the ball WHICH HAD FLOWN over the stands. (Pluperfect tense, something already completed.)

Alternatively use THAT:
The player hit the ball THAT flew over the stands.

These are quite different meanings, and the difference originated from the insertion of one comma. (That's why legal documents don't use them.)

Commas in Lists

Commas are used to separate items in lists:

Bill, Harry, Joe, Fred and Mary all ran down the hill together.

The comma here is replacing the word AND:

Bill and Harry and Joe and Fred and Mary all ran down the hill together.

The comma lightly "puts the brakes on" their run. This kind of list doesn't need a comma in front of the last "and" because it is not replacing it.

Sometimes you will need a comma after the last-but-one item in a list to make it clear that the last two items are not a joined pair:

Egg and cress, salmon and cucumber, ham and chutney, cheese, and scrambled egg, were the fillings on offer.

I'm in two minds whether the last comma needs to be there, myself. I've put it in because it feels like the reader-aloud needs to pause for breath before completing the sentence. However you could equally argue that it's unnecessary.

Commas as voice instruction

Earlier, I wrote:
Reading aloud, when we reach a full stop we allow the voice to drop slightly, which tells listeners that we've completed the thoughts in that sentence.

This also gave clues about how this sentence should be spoken, with pauses after "aloud" and "slightly". It's also another example of "a comma and WHICH" being used to imply a result.

Commas surrounding asides

I am guilty of using too many parenthetic asides (like this one) in my sentence construction, and I have been trying very hard not to do so in this piece. I used to indulge in a lot of brackets (parentheses or braces) but I think I'm on the road to a cure.

I've gone via the comma route, rather than cold-turkey. Wherever you feel desperate to use brackets, try using a pair of commas instead:

Whatever your topic, from love poetry to Doctor Who or Harry Potter fan fiction, Great Writing's online writing group is where you can ...  improve your creative writing.

This works the same as:

Whatever your topic (from love poetry to Doctor Who or Harry Potter fan fiction) Great Writing's online writing group is where you can ...  improve your creative writing.

If the stuff between the brackets or the commas were to be removed, the sentence should still make sense:

Whatever your topic, Great Writing's online writing group is where you can ... improve your creative writing.

This works equally well in writing the "who-said-its" in dialogue. Notice that the commas and full stops go inside the quotation marks, whether single or double:

"Whatever your topic," said Fellpony, "Great Writing's online writing group is where you can ... improve your creative writing."

And that, says Fellpony (parenthetically) is quite enough about both of these jots and tittles.