English nouns, unlike other languages, are made by adding -s or -es at the end of the word. Thus, the place becomes places, and the boy becomes boys.
However, we add -es in words ending with “o” – tomato – tomatoes, volcano – volcanoes, etc.
To make the plural form of some nouns ending with “y” like the sky, fly, and baby, we drop the y, change it to “i” and then we add -es. For example, sky – skies, fly – flies, baby – babies.
Some nouns have irregular plural forms: man becomes men, a woman becomes women, goose becomes geese. The foot becomes feet, the thesis becomes theses, alumnus becomes alumni, etc.
Interestingly, some words have the same form in both singular and plural: “A moose (a kind of deer) is crossing the river. No, wait —three moose are crossing the river!”