In Karin Fossum’s The Drowned Boy, I came across this sentence:
“They each ordered a beer and sat and sipped it in the shade of the trees, looking out at the square and the passersby.”
A search in my online dictionary of choice would indicate that there should be a hyphen in there: passer-by. If there had been one in the translated novel then I would not perhaps have stopped on this sentence and thought about how the noun was made plural. As with every grammar rule there are exceptions, so to make nouns plural is not always a simple case of adding an –s, –es or –ies at the end of the noun.
Passer-by is a compound noun. Sister-in-law and court-martial are two others. When you need to make compound nouns plural you typically (there you go, I’m leaving the rule open for exceptions) make the first noun plural as that (here we go again) tends to be the more important noun within the compound structure: one passer-by versus two passers-by and not two passer-byes (or should that be passer-bies). So its sisters-in-law and courts-martial and not sister-in-laws and court-martials.
There are a few other interesting ways to express plurals. Watch out for that post when I come across a live example from my reading. If, in the meantime, you find interesting compound noun examples that break the rule then please let me know in the comments.