Punctuation 1 - Commas

Commas in written language are used to simplify the understanding for the reader; commas show the reader what idea belongs in what context.

The rules concerning commas in the English language are fairly easy.

Commas are put:

• between two or more main clauses even if the main clauses are connected by a conjunction;
  He bought a car, she bought shoes, they both were happy.
• in between the individual items listed in enumerations. If the last item in the enumeration gets connected to the preceding items by the conjunctions 'and' or 'or', it is optional to put a comma before the conjunction if the items are of the same 'quality';
  Paul cleaned the bathroom, the living-room, the bedroom, the kitchen(,) and the hall.
• after an introductory sub clause;
  While we were living in Sweden, we made a lot of friends.
• in between sub clauses even if they are connected by conjunctions, and the subject of the sub clauses changes;
  Peter needed to put down the shopping bags to unlock the door, because he was fully packed, and Susan was fully packed as well.
• preferably in between a main clause and a sub clause if the subject of the main clause differs from that of the sub clause. A comma in that position prevents ambiguities, thus misunderstandings;
  Peter is going to work by bus today, because / as / since Susan needs the car to run a few important errands this morning.
• after the salutation and the closure in a letter or an email;
  Dear Lucy,
  [. . .]
  Kind regards,
  Peter
• in front of a non-defining relative clause, i.e. a sub clause in which the subject or an object of the preceding main clause gets described in more detail, and that sub clause is not needed for the understanding of the sentence;
  We would like to travel to Rome, which is an interesting city with lots of Ancient sights.
• after an introductory adverb that refers to the entire succeeding sentence;
  Actually, I've never liked you to begin with.
• after the following introductory phrases when they are not used as conjunctions but to connect the idea of the succeeding sub clause to an idea from the previous sentence:
  of course, however, indeed, therefore, nevertheless, nonetheless, moreover, as a matter of fact, besides, for example, for instance, in fact, on the contrary, though, what is more important. If those phrases are inserted into a sentence, you use commas to set them off;
  Nevertheless, he managed to get her an anniversary gift before the stores closed.
  He, nevertheless, managed to get her an anniversary gift before the stores closed.
• to indicate an omission of words;
  His car is black, mine, white. = His car is black, and mine is white.
  Susan had forgotten to make a dessert, so she couldn't offer her guests any homemade sweet treats, hence store-bought ice-cream.
• to set off gerund and participle clauses;
  Having worked all day, Susan was really beat when she got home that night.
• in direct speech to set off the introductory clause;
  Susan asked, “When will you be back?”
• in a, somewhat outdated, way of writing the date in which the 'day' precedes the 'year'.
  Today is July 23, 2007.

It used to be common practice to set off the final 'too' with a comma. Lately, this comma has become optional in everyday practice, it seems.
I packed my swimsuit, my towels, my flipflops, and I should pack the sunscreen(,) too.

Commas are not put:

• in front of any sub clause that gets introduced with 'that';
  I know that you called me, but could I be bothered to pick up the phone?
• in front of infinitive clauses;
  I know it won't be easy to find the solution.
  Paul didn't know what today but [to] wait.
• in front of defining relative clauses;
  The gift which I got you for your birthday was well-meant.
  (= There is more than one gift given in course of the year. Note the difference of meaning in the following sentence:
  The gift, which I got you for you birthday, was well-meant.
  In the above sentence with a non-defining relative clause the speaker says that only one gift is given and adds the additional information that it is a birthday gift.)
• in front of if-clauses if the main clause precedes the if clause;
  I'll call you later if I have the time.
• in front of indirect questions;
  I cannot tell you when his birthday is.
  Paul wants to know whether you'll visit us for dinner tomorrow.
• in front of adverbial clauses. Commas are optional after adverbial clauses.
  Call me after you

get home.
After you get home(,) call me.