The 'y' to 'i' Spelling Rule
beauty → beautiful
happy → happiness
angry → angrily

We change the "y" to " i" when adding suffix endings if a word ends in a consonant + Y.
*But we don't change it when we add "i" endings like -ing, -ish which already begin with an "i")

beauty + ful → beautiful. Also beautify, beautician
happy + ness → happiness. Also happily, happier, happiest
angry + er → angrier. Also angriest, angrily
pretty: prettier, prettiest
ready: readily, readiness
dry: dried, but drying, dryish (keep the “y” because we don’t want two i’s together) but exceptions are dryer or drier, dryly or drily
defy: defies, defied, but defying
apply: applies, applied, application but applying

Some "a" + "y" words change the "y" to "i" check out these -ai- patterns in
day + ly = daily,
gay+ ety = gaiety. Also gaily
lay – laid
pay – paid
say - said
slay - slain

We have a reversal of "y" to "i" rule when we change some words ending in “ie” to “y”
die + ing = dying (dieting X too many vowels in a row!)
tie + ing = tying
lie + ing = lying

When we add -s to a consonant + y we change the "-y" to "-ies".
apply - applies
supply - supplies
deny - denies
magnify - magnifies
dry - dries
balcony - balconies
ceremony - ceremonies
story - stories

Macmillan Dictionary online says: Get it right: happy
Although the adjective happy ends in the letter 'y', the related noun happiness is spelled with an 'i' (not a 'y'):
✗ Health, love, and happyness are what everyone looks for in their life.
✓ Health, love, and happiness are what everyone looks for in their life.

For most other adjectives ending in 'y', the same rule applies:
adjective → related noun
empty → emptiness
lazy → laziness
lonely → loneliness
tidy → tidiness
ugly → ugliness

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Y To I Rule