Nouns ending in "y" preceded by a vowel form their plurals by adding "-s".
Here are some examples:
* boy - boys
* toy - toys
* monkey - monkeys
* valley - valleys
The rule applies regardless of whether the vowel is a single vowel or a diphthong:
* quay - quays
* journey - journeys
However, if the "y" is preceded by a consonant, the plural is formed by changing the "y" to "ies":
* baby - babies
* city - cities
* story - stories