This is a very simple thing that can make the difference between your writing looking professional or amateur. How do you take one item and indicate that you now have more than one? In other words, how do you make it plural?
You add an “s.” Simplest thing in the world. And if the word already ends in “s,” or something a little odd like “x,” then you add “es” instead. There are a few weird plurals out there that you do have to learn differently, but they are a very small minority of words.
For the vast majority of words, it’s exactly that simple: add “s” or “es” to make them plural. So if you had one “pen,” then more than one is “pens.” One “rock” becomes many “rocks.” One drinking “glass” becomes many “glasses.” An “ax” (or “axe”) becomes several “axes.”
Isn’t that easy? And yet for some reason, more and more in recent years, people have started trying to make plurals by adding an apostrophe before the “s.” As in “pen’s” or “rock’s.” But this is absolutely wrong, and if you do this, it will be the first signal to your reader that you really don’t know how to write properly.
Why has this problem been growing lately? My theory is that it stems from the more complex use of the apostrophe to create a possessive. (As in “The bird’s nest,” meaning “The nest that belongs to the bird.”) Even though the simple possessive is made just by adding an apostrophe and an “s,” it gets a bit more complicated when you’re creating a possessive of a word that’s plural. (So for many birds, it would be “The birds’ nest.” The apostrophe added after the existing plural “s.”)
What I think has happened is that people are “extrapolating sideways.” They don’t fully grasp how to use the apostrophe and “s” in combination for possessives, and therefore get the vague feeling that any time you add an “s,” there’s got to be an apostrophe in there somewhere.
But no. You can help yourself by remembering that plurals are very simple. Leave possessives off to the side for now. For a plural, just add an “s” or “es,” and you’ve got the majority of them covered, very simply. Then you can learn the slightly different plurals for some exceptional words like thesis (plural: thesis), axis (plural: axes), or datum (plural: data).
But for most words, the “s” — without an apostrophe! — is the easy, simple way to make something plural.
(For a great explanation of when to use — or not to use — an apostrophe, have a look at The Oatmeal website, and this great comic: How to Use an Apostrophe. And if you’re on Flickr, and would enjoy seeing countless photographic examples of the misuse of apostrophes in creating plurals, you might want to join the Group, Atrocious Apostophe’s.)