When a word ends in a consonant followed by y, change the y to i before adding most suffixes — that's why 'baby' becomes 'babies,' not 'babys.' When a vowel comes before the y, nothing changes.
The rule in one sentence
Look at the letter right before the final y. If it's a consonant, change y to i before adding a suffix (happy → happiness). If it's a vowel, leave the y alone (play → played). The one big exception: the y never changes before -ing.
The -ing exception
Never change y to i before -ing
Even after a consonant, y stays y before -ing — cry becomes crying, try becomes trying, study becomes studying. Changing the y here would create an awkward double-i spelling ("criing"), so English simply keeps the y in this one case.
- Find the letter right before the final y. Consonant or vowel?
- Consonant + suffix (not -ing) → change y to i. Happy becomes happiness, happier, happiest.
- Any suffix is -ing → keep the y no matter what. Study becomes studying, never studing.
- Vowel before y → always keep the y. Stay becomes stayed and staying, never staid.
Proper nouns are the other exception
Names and other proper nouns don't follow the consonant-before-y rule, even when a consonant sits right before the y. A girl named Mary keeps her name as Marys, not Maries, when talking about more than one Mary in a class. The same goes for Emily, Betty, or Wendy — "there are two Emilys in my class" is correct, not "two Emilies." Days of the week that end in a vowel before y (Monday, Friday) already follow the normal vowel rule and simply add -s (Mondays, Fridays), so those were never an exception in the first place — but names like Mary and Betty are a genuine departure from the pattern taught for common nouns like baby and happy. The reasoning is more about convention than sound: changing someone's actual name to a different spelling, even just to add a suffix, feels wrong in a way that changing "baby" to "babies" doesn't. It's worth mentioning to kids directly, since this is exactly the kind of case where a child who has confidently learned the rule will confidently misapply it — writing "there are two Marys" as "there are two Maries" is a very natural, rule-following mistake, not carelessness, and it's worth praising the reasoning even while correcting the spelling.
This is one of the two main rules for adding suffixes to a base word — the other covers silent-e words in the drop-the-e rule. See the full suffix spelling rules guide for how all of these fit together, including plurals in our plural rules guide.
Free printable for this topic
Download the Suffix Spelling Rules Cheat Sheet — a free PDF you can print and use today.