Look at these spelling changes as nouns change from singular to plural: house to houses; dress to dresses, boy to boys and activity to activities.
For nouns to change from singular to plural in the English language, several forms are taken. Some take “s”, “es”, “ies”, and etcetera.
Some learners are ambitious enough to master the spelling of each word they come across; some are also clever enough to learn spelling by mastering the rules.
Nouns that end in “y” have two alternatives suffixes when changing to plural: either adding “s” or “ies”.
For example: key changes to keys, baby to babies, day to days, lady to ladies and many more.
The rule simply depends on the letter before the last “y”.
When the letter before the last “y” is a vowel (a, e, i, o and u), the noun takes “s” to become plural.
This is why valley changes to valleys; journey to journeys, essay to assays, monkey to monkeys, toy to toys, way to ways and survey to surveys.
But when the letter before the last “y” is a consonant (all other alphabets apart from a, e, i, o and u), the noun takes “ies” to become plural.
For instance: company changes to companies, country to countries, duty to duties, enemy to enemies, fly to flies, rally to rallies, secretary to secretaries, sky to skies, story to stories and many more.
Some exceptions to this rule are words such as colloquy (meaning meeting or discussion). The word takes “ies”, colloquies” even though the letter before the last “y” is a vowel “u”.
The reason behind this odd spelling is that “qu” in English is always a single sound. There is no English word where “q” is written without “u”.
This means the “u” before the “y” in colloquy is not considered a vowel and so, the plural should be colloquies. Otherwise, another word with “u” will follow the rule such as “guy” becoming “guys” and “buy” becoming “buys”.
Other odd words in the rule can be proper nouns such as July and names of families.
We write “Julys” are hot months and the “Kennedys” (the family of Mr and Mrs Kennedy) are good people.
The writer is a professional English Language instructor