Since most writing manuals (Chicago, MLA, APA, etc) change their rules slightly every year, there’s less and less point to memorizing specific rules for comma placement. Instead, read each sentence out loud. When you pause, add a comma.
The point of a comma, beyond separating phrases and clauses, is to give the reader a sense of how the author wanted the sentence to sound. This is another prime reason for comma flexibility–some are, in fact, optional! Separating parts of a sentence via comma places added emphasis on each part. If an author doesn’t want that emphasis, often times the comma can be removed.
Commas slow down a sentence, so when you want the reader to savor each phrase/clause, more (appropriately placed) commas are important. When you want the sentence to be read quickly, to reflect a characters racing thoughts or heartbeat, or to mimic more natural human dialogue, commas can be omitted–most of the time.
That being said, some commas are essential. We’ll review each situation below:
When I went to the store I forgot to buy Trojan Magnum Condoms which put a dent in my weekend plans.
Version 1: When I went to the store [pause] I forgot to buy Trojan Magnum Condoms [pause] which put a dent in my weekend plans.
Version 2: When I went to the store I forgot to buy Trojan Magnum Condoms [pause] which put a dent in my weekend plans
Incorrect: When [pause] I went to the store I forgot to buy [pause] Trojan Magnum Condoms [pause–correct] which put a dent in my weekend plans.
The first version places more emphasis on “I forgot to buy…” because it’s offset from the rest of the sentence by commas. The second version places the most emphasis on “which put a dent in…” because it follows the only pause.
The incorrect version is reflective of when many people place commas in a sentence. When read out loud, it’s obvious the pauses are not correct; when most people write, however, they throw commas in where they THINK they’re supposed to go, not where they help the sentence cadence.
When, I went to the store I forgot to buy, Trojan Magnum Condoms, which put a dent in my weekend plans.
This may look correct. The writer may know the sentence needs commas. But they are not in the correct places.
Stop thinking about commas as following strict rules you must memorize. Start thinking of commas as representations of pauses when reading a sentence out loud.