Do nouns ending in y preceded by a vowel form their plurals adding s?

Yes, 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 is that you add "s" to the end of the word if the "y" is preceded by a vowel (a, e, i, o, u). If the "y" is preceded by a consonant, you change the "y" to "ies" to form the plural.