When does Y change to IE?

Most words with an "i" sound at the end end in -Y
The plural of nouns which end in -Y changes to -IES

a lady; many ladies
my family; many families
1 granny; both grannies
one city; all the cities
one fatality; many fatalities
the gallery; all the galleries
one theory; many theories
one pony; lots of ponies

If the -y is preceded by a vowel though, the plural is -ys

day; days
toy; toys
monkey; monkeys

You can put an ie at the ending of a word as a sign of affection

What a cutie!
She's a sweetie

Note - no apostrophes for plurals.

Comparatives and superlatives of adjectives which end in -Y

silly, sillier, silliest

funny, funnier, funniest

happy, happier, happiest

merry, merrier, merriest

Note - no apostrophes for comparatives and superlatives of adjectives!

Are you seeing a pattern here?

Apostrophe only 

1  for possession

Mary's brother

the pony's saddle

2 for a missing letter

Mary's late = Mary is late

don't be late - do not be late