The importance of commas is, at times, neglected. However, in the context of a formal piece of writing, whether it be an essay, a business letter, or a report, commas take on a greater importance. Commas placed in the wrong position or omitted can cause cohesive problems and misunderstanding.
It is important to note that the interpretation of punctuation rules can vary and depending on the reference style and publication guideline, there can also be variances. The rules supplied here are mainly based on Straus' Blue Book of Grammar and Punctuation. Please note that the rules are not listed in any particular order, so the first rule is not necessarily more important or central than the fifth one, etc.
Commas between modifiers
Use a comma to separate two premodifiers in a noun phrase, when the word and can be inserted between them (but not when the first modifier is descriptive and the second one is classifying):
(1) He is a strong, healthy man.
(2) We stayed at an expensive summer resort.
You would not say
(3) We stayed at an expensive and summer resort
so no comma. However, you would say
(4) He is a strong and healthy man.
Please note that a classifying modifier tells us which class (the referent of) a noun belongs to, for instance, in this particular case, that it is not any type of resort, but a summer resort. A descriptive modifier is an adjective (phrase) that tells us what the noun (or the combination of the classifying modifier and the head noun) is like.
Commas and titles
Use commas before or surrounding the name or title of a person directly addressed:
(5) Will you, Aisha, fill in that form for me?
(6) Yes, Doctor, I will.
Please note that you should capitalize the title when you are directly addressing someone.
Months and years
Use a comma to separate the day of the month from the year:
(7) Anna met her husband on December 5, 2003, in Lund, Sweden.
We often also add a comma after the year, if it is the last word of an adverbial of time, especially if we want to separate it from another adverbial, as in our example here (where it is separated from the place adverbial in Lund).
Degrees, titles, and names
Use commas to surround degrees or titles used in connection with names:
(8) Al Mooney, M.D., knew Sam Sunny Jr. and Charles Starr III.
Please note that commas are not required around Jr. and Sr., and should not be used to set off II, III, and so forth.
American cities and states
Use a comma to separate the city from the state and after the state in a document:
(9) I lived in San Francisco, California, for 20 years.
If you use the two-letter capitalized form of a state in a document, you do not need a comma after the state:
(10) I lived in San Francisco, CA for 20 years.
With addresses on envelopes, the following format is generally used:
(11) San Francisco State University
1600 Holloway Avenue
San Francisco, CA 94132
Commas and sentence flow
Use commas to set off expressions that interrupt sentence flow.
(12) I am, as you have probably noticed, very nervous about this.
An alternative to using commas here would be to use dashes:
(13) I am—as you have probably noticed—very nervous about this.
Commas and dependent clauses
Generally speaking, when starting a sentence with a dependent clause, use a comma after it. Conversely, do not put a comma before the dependent clause when it is found at the end of the sentence:
(14) If you are not sure about this, let me know now.
(15) Let me know now if you are not sure about this.
After sentence introductions
Generally speaking, a comma is used after the introductory part of a sentence, whichever form this introductory part has:
(16) To apply for this job, no previous experience is required.
(17) On February 14, many couples give each other flowers.
(18) However, this is actually not true.
(19) Before breakfast, you'd better not ask such difficult questions.
By the introductory part of a sentence, we should understand a clause element that precedes the subject in a non-inverted structure, or one that precedes the first auxiliary in a partially inverted structure. Most of these introductory parts will be adverbials of various types, as in the examples above.
It would be a lie to actually claim that all (good) writers always put a comma after any type of introductory part of a sentence. To some extent, whether or not a comma will be used depends on the length of the introductory part. A rule along the following lines has even been suggested:
Use a comma after phrases or clauses of more than three words that begin a sentence (unless it is the subject of the sentence). If the phrase has fewer than three words, the comma is optional.
Whether or not a comma is used also depends on the type of the introductory element, and, last but not least, on whether or not there would be a slight pause if the clause were read out loud.
Remember that the main reason we use commas in the first place is to help our readers understand our sentences in the exact ways in which they are intended to be understood. If commas will help them do this, using them could be considered to be correct, regardless of any prescriptive rule that some expert may have formulated.
No comma after the subject
Generally speaking, never put a comma after the subject of a sentence, regardless of the length of the subject:
(20) [The people we met in Alberta] loved wild animals.
(21) [Some of the material on the AWELU platform and some of her general ideas on punctuation] might be summarised elsewhere.
(21) [That the president was such a funny guy] surprised Laura Palmer.
If the subject is realised by a noun phrase, the last part of which is a non-restrictive relative clause as in example (22) below, some other non-restrictive postmodifier, such as a non-restrictive reduced relative clause (23), or a restrictive appositive phrase (24), the subject will be followed by a comma anyway, as the examples below are intended to illustrate.
To rescue the general rule, we could argue that in these cases, it is the postmodifier, rather than the subject itself, that is followed by a comma, but these apparent exceptions still deserve to be pointed out to writers.
(22) [Some of my students, all of whom take their studies seriously,] failed.
(23) [His boyfriend, as charming as usual,] open the door for us.
(24) [My brother, Bert,] used to run a record company.
Please remember that whether a modifier is restrictive or non-restrictive in a given sentence sometimes depends on what the world is like. For instance, if I had more than one brother, one of whom was called Bert, (24), with a non-restricive apposition surrounded by commas, would not be OK. Instead we would make it restrictive and remove the commas.
Commas and non-restrictive postmodifiers
If something or someone is sufficiently identified, the description that follows is considered nonessential and should be surrounded by commas:
(25) Freddy, who has a limp, was in an auto accident.
Freddy is named, so the description is not essential for the identification of Freddy.
(26) The boy who has a limp was in an auto accident.
We do not know which boy is being referred to without further description; therefore, no commas are used.
In a sense, this rule states what is mentioned elsewhere as an apparent exception to the rule that subjects are never followed by commas (see link below). However, this rule about commas and non-restrictive postmodifiers also points out that this generalisation does not only concern subjects.
Commas and coordinating conjunctions
Use a comma to separate two main clauses joined by one of the coordinating conjunctions and, or, but, for, so, yet, and nor:
(27) There's no one in the house, and the garden seems to be deserted as well.
(28) She will have to be fired immediately, or I will leave this firm now.
(29) I have painted the entire house, but he is still working on sanding the doors.
(30) They wanted to hire another senior lecturer, so they advertised a post.
At least in the case of the conjunction and, we normally omit the comma if the clauses are both really short
(31) I paint and he writes.
or if the sentence is otherwise totally impossible to misunderstand.
If the subject is identical in the two conjoined main clauses and omitted from the second one, we do not insert a comma before the conjunction:
(32) He thought quickly but still did not answer correctly.
Commas and tag questions
Use a comma to separate the statement from the tag question:
(33) I can go, can't I?
Contractions (such as can't) are avoided in academic writing, and so are normally tag questions and direct questions in general (that is, we normally use indirect questions instead).
Contrasting parts of a sentence
Use a comma to separate contrasting parts of a sentence.
(34) That is my money, not yours.
Before introductory words
Use either a comma or a semicolon before introductory words such as namely, that is, i.e., e.g., for example, or for instance, when they are followed by a series of items. Also put a comma after the introductory word:
(35) You may be required to bring many items, for example, sleeping bags, pans, and warm clothing.
(36) You may be required to bring many items; for example, sleeping bags, pans, and warm clothing.