Commas serve to separate or link things. Some of the things might be
- a list of adjectives attached to a noun
a balmy, beautiful, sunshiney, brilliant day. Note that the last item in the list has no comma because it’s the main adjective modifying the noun day. The rest of the adjectives are extras and have to be spliced in by the commas.
- ideas (words) inserted into a sentence
A basic sentence (subject-verb-object) can be added to in many ways. It’s possible to insert extra information about some part of a sentence without changing the structure of the sentence simply by inserting that information between commas:
I ate, without previously thinking I was hungry, the whole of the pie.
In simple examples like this it is possible to see the functional similarity between a pair of commas [, ,] and parentheses [ ( ) ]. That’s not a coincidence. They are effectively and functionally identical.
- ideas (words) added to the start or end of a sentence
Simply adding to the basic sentence by tacking on ideas or words at the beginning or end is easily done by using the comma as a kind of glue:
Without previously thinking I was hungry, I ate the whole of the pie.
I ate the whole of the pie, without previously thinking I was hungry.
In the second example, if the comma is not used the focus of the compound sentence is shifted from the act of eating to the act of thinking, or alternatively it makes both acts equally important. Little differences matter. The art of choosing whether to use a comma or not is alive and well in the legal profession where a stray or absent comma in a document may be responsible for a unilateral income-sharing lesson of somewhat rueful kind.
For the more grammatically adept, let’s expand some of the ideas above:
- information placed in apposition
Grammarians say information inserted in a form which might replace a noun is placed in apposition to the noun. The alternative data is inserted using the [, ,] format. The alternative data does not disturb the sentence itself.
I, Joan, ate the whole of the pie.
The test of whether something is in apposition is to delete the commas and the information between them and check that the sentence is still whole, complete and entire. A further test is to let the comma-ed* information replace the noun it is put beside and to check again that the meaning of the sentence is unchanged and complete.
- phrases in a sentence
Phrases are groups of words which have an understandable meaning, but miss out on being sentences because they have no verb. Phrases only get connected to a verb if they are added to a sentence (which does have a verb). Commas make the connection, but also separate phrases out from the main sentence. They have no life of their own:
I ate, noisily and with some satisfaction, the whole of the pie.
- clauses in a sentence
Clauses are meaningful groups of words built around a verb. A sentence is a clause on its own, but other clauses can be tacked on if they deal with the same topic. It’s a kind of shorthand. Here’s an example which is more complex than it looks: it takes longer to explain than to read:
I ate with some satisfaction(1) and complimented the cook(2), telling her how perfect her cooking was(3).
(1) main clause capable of standing on its own two feet and forging an independent and high-flying career: just insert a full stop and check it for yourself.
(2) subordinate clause which borrows the subject of (1) to make sense: the conjunction and is enough to make the link in our heads: no comma needed at all. The clause hasn’t got enough information to be a sentence on its own and will always be dependent on mummy or daddy main clause. Bummer.
(3) another subordinate clause needing the reader to mentally connect up the meaning of the first two clauses before making sense: this time, a comma was chosen to make the link, although another conjunction would have done the same job just as well and instead. Here it’s a matter of style, not grammar.
- Replacing the colon and the semi-colon
Woe to the colon and it’s half-sib, for they are sorely misunderstood and cast out… If you’ve had a good look around this site, you will know that I have an affection for these punctuation marks. What’s more, I was taught how to use them correctly. I might be of the last generation to be able to do so without further specialist tuition. Suffice to say here that without some understanding of the structure of a complex sentence (meaning of main, subordinate, and coordinate clauses and the phrasal structures within) the use of colons and semi-colons will be a mystery to you. That’s not an insult: for most people it’s entirely irrelevant.†
Because there is no longer any point in bothering to unravel grammatical structure, it has become acceptable to replace colons and semis with commas. I’m mostly okay with that, provided that writers don’t overreach themselves by writing long, complex sentences which are invariably badly-punctuated because they don’t understand the complexity they are dealing with. One only has to look at how newspaper articles are now written – paragraphs are commonly one sentence – to see the trend. Short sentences require less punctuation. The chances of making a mistake diminish in proportion to the number of words. Stylistically inelegant as it may be, and an affront to the rhetoricians in our past shared history, it is nevertheless an acceptable solution to a tricky problem. Be clear. Be brief. Be sparse in punctuation and use any internal punctuation sparingly, especially the comma. If in doubt, leave it out: you’ll make fewer mistakes that way.
*What – you expected me to apostrophize, as in “comma’d”? Or to plunge heedlessly on with “commaed”? Nothng that ugly on this website, Buddy. I’d sooner make it up as I go along and have something I can live with at the end of the day. † Just to say, go and read some 19thC writers and boggle at the sheer length and complexity of their sentences. I swear that I once read a bewildering Charles Dickens sentence that went on for more than a page before the full stop sauntered into view and ended the mental agony. They had the learning then, right enough, but could they tweet or Facebook?