How did English, French, and Spanish all get to make plurals by adding an "s", when other Germanic and Romance languages have different rules for forming plurals?

Answer 1

I'm not equipped to tell you why English has plurals ending in -s, though I'll hazard a guess later. I can explain why French, Spanish, and other Romance languages do what they do to form plurals.

Latin had a case system. It was more complicated but dissolved over time. In the end, it had a nominative (subject case) and a non-nominative (non-subject case). Each noun, then, could be singular nominative, singular non-nominative, plural nominative, or plural non-nominative. Let's use casa 'house' as our sample word.

Latin casa

singular: casa (nom.) / casa (non-nom) - yes, these forms were the same

plural: case (nom.) / casas (non-nom)

What happened? There's a geographical divide (the Spezia–Rimini Line). Languages to the west of this line share certain characteristics. These languages include French, Occitan, and the languages of the Iberian Peninsula (Spanish, Portuguese, Catalan, Galician). To the east, there's Italy, Sardinian, and Romanian, to name a few. Western Romance took its plural from the non-nominative case, whereas Eastern Romance took its plural from the nominative case.

Now, if I had to guess, English has -s plurals due to French influence. I'll also add that German Kuli does, in fact, form its plural with an -s, Kulis. That said, German has many ways of forming plurals, and -s is not the most common one.

In answering your question, I referenced Romance Languages (Alkire & Rosen, 2010). It's a college-level textbook, but if you're interested in the history of Romance languages, you might want to check it out.