A long time ago in a school district far, far away, a bunch of high school students flunked a particular section of the high school competency test--the test they needed to pass in order to receive a high school diploma rather than a Certificate of Attendance. In fact, so many students flunked this particular section of the test that the entire student body (more than 2000 students) endured periodic remediation sessions on the subject--in every class. At a certain time every week, every class would stop what it was doing and every teacher would present a short and simple lesson on the correct way to form possessives of plural nouns. A visitor to the building might have been surprised to see the same lesson being presented in French classes, driver's ed classes, art classes, and shop classes: "To form the possessive of a plural noun, first write down the plural form of the noun; if it ends in s, add an apostrophe, and if it does not end in s, and 's."
It seems so simple! And yet today I have on my desk a pile of papers dealing with the playwright Suzan-Lori Parks, and so far I have seen the following spellings: Parks, Park's, Parks', Parks's, Parkses, and Parkses'. Some of those are correct in certain contexts, but they are being used in these papers almost interchangeably. So I did a little remediation at the beginning of class today, drawing on the valuable lesson I learned in every high school class I ever took: to make a noun plural, add s or es (no apostrophe). To make it possessive, add 's. To make a plural noun possessive, see the paragraph above. So if we are studying a play by Suzan-Lori Parks, it is Parks's play, but she might be related to a bunch of other Parkses who all live in the Parkses' house. See? Easy as pie.
So I guess I should be grateful to my driver's education teacher: I may have flunked parallel parking, but at least I know where to park an apostrophe.