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This tends to be more applicable in American English, though it is still recommended in British English for longer introductory clauses.
Often, as editors and writers, we join sentences together with the word "and". However, frequently I see commas placed before the word "and" when it isn't warranted as well as sentences that need this comma.
I raise this issue enough at GAN to make an explainer page. The sentences are from articles I've reviewed at GAN.
The trick is to split the sentence by removing the "and". If you have two separate standalone sentences, you need a comma.
When you need ", and" or a semicolon
For instance, take this from a GA I reviewed:
The circle itself, called Memorial Circle at the time, was made into a park in the early 1870s and Luther Place Memorial Church was built on the north side of the circle in 1873.
We get two sentences:
The circle itself, called Memorial Circle at the time, was made into a park in the early 1870s. / Luther Place Memorial Church was built on the north side of the circle in 1873.
Each of these sentences has a subject and a verb: "circle / was made" and "church / was built". Thus, when combined, they need to either have ", and" or a semicolon between them.
The circle itself, called Memorial Circle at the time, was made into a park in the early 1870s, and Luther Place Memorial Church was built on the north side of the circle in 1873.
The circle itself, called Memorial Circle at the time, was made into a park in the early 1870s; Luther Place Memorial Church was built on the north side of the circle in 1873.
When you need "and" without a comma
This also works the other way. Here's a sentence from another GA:
San Diego won, and gave Buffalo tackle Dick Hudson and two future draft picks in exchange for Rote's signing rights.
If we split this sentence that has ", and" as a linkage, we get:
San Diego won. / Gave Buffalo tackle Dick Hudson and two future draft picks in exchange for Rote's signing rights.
The second portion lacks a subject and is not a complete sentence. Thus, if you do not wish to add a subject to the second portion, you leave out the comma.
San Diego won and gave Buffalo tackle Dick Hudson and two future draft picks in exchange for Rote's signing rights.
Other conjunctions
This is also the case for "but":
Oakland had finished the previous season at a league-worst 1–13, but emerged as serious rivals to the Chargers in the AFL West during Davis's first season there.
If we split this sentence, we get the missing-subject fragment emerged as serious rivals to the Chargers in the AFL West during Davis's first season there.. To correct this, we simply remove the comma (or add a subject, as in #2—note the bolded addition):
Oakland had finished the previous season at a league-worst 1–13 but emerged as serious rivals to the Chargers in the AFL West during Davis's first season there.
Oakland had finished the previous season at a league-worst 1–13, but the team emerged as serious rivals to the Chargers in the AFL West during Davis's first season there.
Balanced comma exception
Of course, sometimes balanced comma considerations require a comma even when its placement would otherwise not be called for. These are correct:
The station was issued a construction permit on January 1, 1960, and signed on that August.
KOB began broadcasting from Las Cruces, New Mexico, before moving to Albuquerque.
You'll note that "signed on that August" and "before moving to Albuquerque" are not separate sentences. However, MOS:DATECOMMA requires that we have a comma to balance the appositive contained in "1960", and MOS:GEOCOMMA orders the comma after "New Mexico".
