Question

why plural of baby is babies and monkey is monkeys

Answer 1

because when you replace y to convert it into pural form it becomes ies
and in same in monkey

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Answer 2

Why Is There Confusion over the Plural of Monkey?

Monkeys is the only way to make the noun monkey plural. ... When a noun ends with [vowel] + y, you only need to add an "s" to form the plural