Cats, buses, and babies all mean "more than one" — but they're spelled three different ways. Here are the 3 rules that predict which spelling to use, plus the short list of irregular exceptions.
Rule 1: Just add -s
The default rule, covering most words.
Just add -s
catsdogsbookschairstreesbirds
Rule 2: Add -es after s, x, z, ch, sh
These endings already end in a hissing or buzzing sound, so English adds a full extra syllable (-es) rather than just -s, which would be impossible to say clearly.
Add -es
boxesbusesbuzzeschurchesdisheswishesfoxes
Rule 3: Y endings — depends what comes before
| Before the Y | Rule | Example |
|---|---|---|
| A vowel (ay, oy, ey) | Just add -s | toy → toys, day → days |
| A consonant | Change y to i, add -es | baby → babies, city → cities |
Y → IES (consonant before y)
babiescitiespuppiescandiesfamiliesponies
A small set of irregular plurals
child/children, mouse/mice, foot/feet, man/men, tooth/teeth — these don't follow any of the regular rules and are best taught as a short memorised list rather than an attempted rule.
Plurals are one more piece of the spelling-rule picture — see our 10 English spelling rules and suffix spelling rules for the rest.