A plural means more than one. One cat is singular. Two or more is plural: two cats. Watch out: after two, three, four… don't forget the ending — three cats ✓, not three cat ✗.
What a plural means
one cat → two cats
one book → some books
one apple → many apples
Try it: 1 book. Now five ___. (books / book / a book) Answer: books — Five is more than one, so add -s: five books.
Most nouns: add -s
Look at the pattern: book → books · car → cars · dog → dogs. Most nouns just add -s.
book → books
car → cars
dog → dogs
Try it: I have three ___ at home. (dogs / doges / dog) Answer: dogs — Most nouns just add -s: dog → dogs. There is no extra e here, "doges" adds a sound that is not needed.
After s, z, x, ch, sh: add -es
When the noun ends in s, z, x, ch, or sh, the ending is -es: bus → buses · box → boxes · watch → watches.
bus → buses
box → boxes
watch → watches
Try it: 1 box. Now two ___. (boxes / boxs / box) Answer: boxes — You had the right idea adding an ending, after -x it is -es, not -s: box → boxes.
Try it: Remember the ending. She fixed two ___. (watches / watchs / watch) Answer: watches — "Watch" ends in -ch, so the plural is -es: watches. (After s, z, x, ch, sh the ending is -es.)