…and no, that's not a spelling error.

The comma  — that tiny little squiggly thing one sometimes sees between words — can be used, amongst many other things, to mark an interjection. For example:

“Now, are you ready to go?”

implies that the speaker is trying to get the putative listener's attention, the rough — but slightly more polite — equivalent of:

“Hey, you! Are you ready to go?”

whereas:

“Now are you ready to go?”

implies a level of impatient sarcasm, which might also be indicated with italics:

“Now are you ready to go?”

with the more or less hidden subtext being:

(‘I'm tired of waiting for you,’ so) now are you ready to go?

Like the jot or tittle which shall never pass from the Law, those little commas mean quite a lot, and can be omitted — or included — only at the writer's peril. It helps a lot to speak one's words aloud as one writes, or at least maintain an active mental awareness of how one means the words set down on paper to be spoken.

The comma should also be omitted when a word is used as a simple noun, such as when the word ‘now’ refers simply to the present time without any hint of sarcasm:

“Now is the winter of our discontent….”

Appositives Attract

An appositive is a word or phrase equivalent in meaning to an adjacent word or phrase, but it's sometimes difficult to determine whether that word or phrase is actually being used appositively or not, although one can always tell if you actually hear the sentence spoken aloud.

“I talked to my sister Karen yesterday.”

Is not an apposition, because in fact I have two sisters and ‘Karen’ is being used as a type of postpositional adjective to distinguish my sister Karen from my sister Barbara.

If I had only one sister, I would write:

“I talked to my sister, Karen, yesterday.”

because ‘my sister’ and ‘Karen’ would mean exactly the same thing, with the ‘Karen’ added only to clarify the meaning of ‘sister’ if it had slipped the listener's mind for any reason, just as one might say:

“Have you seen my (only) book Winnie the Pooh?”

assuming that I don't have more than one book. Of course one could use ‘Winnie the Pooh’ in a non-appositive manner as well:

“Have you seen my book, Winnie the Pooh (as opposed to my other book, Now we are Six) ?”

Just Because

English is a remarkably subtle language. Not for us the tedious and myriad declensions and cases so common in most human languages, so much of our meaning — at least in spoken discourse — is implied by tones and pauses. Unfortunately, tones and pauses are usually implied by commas — or their absence — in written dialogue, so their putative presence — or absence — isn't inherently obvious without at least mentally hearing the sentence in your imagination. ‘Because’ can introduce sentences with or without what we call a ‘dependent clause’ in English, although the actual words might be exactly the same. The only difference lies in the punctuation.

I don't hate him because he's stupid.

I don't hate him, because he's stupid.

mean two completely different things. The first says:

I hate him, but not because he's stupid.

whilst the second says, more or less:

I don't hate him, because because the fact that he's stupid makes me pity him.

In this case, the comma is a marker of dependency and mirrors the distinction we make between the two meanings when we speak, inserting a pause before dependent clauses.

Relatively Speaking — Which and That

We make a distinction in English between parenthetical phrases and relative phrases, the difference between them being only what's being emphasised in the sentence proper.

“The cow that jumped over the Moon has absconded with the milk money.”

This sentence uses the words ‘jumped over the Moon’ to identify a particular cow, so it might as well be “the red cow.” Because we're emphasising the cow, we omit commas, because the descriptive phrase is necessary to distinguish this particular cow from all others.

“The cow, which jumped over the Moon, has absconded with the milk money.”

In this version of the sentence, we're primarily concerned with the milk money, not the cow, so the phrase ‘jumped over the Moon’ is parenthetical, not relative, and commas here stand in for parentheses, and are necessary, but might also be replaced with real parentheses:

“The cow (which jumped over the Moon) has absconded with the milk money.”

The intent is not to identify a particular cow, but to explain the sorts of exploits that particular cow gets up to. Tch, tch, she was always a little daring, so no wonder she stole the milk money and ran off to parts unknown.

Introducing, the Phrase

People often begin sentences with rather arbitrary words and phrases. In some cases, it's de rigueur to set off the phrase or word with a comma, if only because most people pause slightly before continuing. In other cases, it's arbitrary, because at least some people don't pause between the introductory phrase and the rest of the sentence.

“Nevertheless, when the weather is warm, I like to reach right up and touch the sky.”

“In the summer time when the weather is high you can reach right up and touch the sky….”

The first sentence uses a word which must be followed by a comma. For the sake of consistency, many purists insist that all such dependencies should be set off with a comma. It's entirely possible to make perfect sense of the lyrics of the song without any commas at all, however, so it's quite common in casual writing to omit them. Be aware, however, that at least some purists will grit their teeth and mumble bitter imprecations about the sorry state of the modern educational system, so would be happier to see:

“In the summer time, when the weather is high, you can reach right up and touch the sky….”

Independent Thinking

Many sentences are made up of two or more independent clauses separated by a conjunction, a word which is rather circularly defined as a word used to join two independent clauses. These are often set off by placing a comma before the conjunction, such as:

“Oh, I am a Cook, and a Captain bold,
And the Mate of the Nancy brig,
And a Bo'sun tight, and a Midshipmite,
And the crew of the Captain's gig.”

When the independent clauses are short, as they are in the above song lyrics, it's fairly common to omit one or more of these distinguishing commas, but not strictly correct.

If one worries about the Grammar Police, who have no actual authority unless one is working for a stickler for such things, you might want to include them, but even if you omit them be aware that there's a completely arbitrary point at which most people will include a comma to avoid confusion. The exact point at which people become confused will vary by the capaciousness of their short-term memory, so it's possibly better to err on the side of conservatism.

Coördinating Your Adjectives

Some adjectives come in integral pairs, and some do not. Those whose relationship is defined by their pairing should never be separated by commas. Those which are completely independent should always be separated by something, most often a comma or the word ‘and.’

“I looked up at the dark blue sky.”

‘Dark blue’ defines a particular colour, so the two adjectives are coördinate, so cannot be separated by a comma, or anything else.

“Black and blue” doesn't define a particular colour, so these two words are noncoördinate, so should always be separated by either a comma or the word ‘and.’

You can always tell the difference by plugging in the word ‘and,’ so all you have to do is listen as you write.

“I had on a dark blue jumper, black and blue striped trousers, and a red scarf.”

See also my blog on the vocative comma in English.