Here's the rule: If there is a vowel (a e i o u) before the letter 'y', then you just add an 's' in the normal way. For example: boys, days, monkeys, and so on. If there is a consonant ( b c d f etc) before the letter 'y', then you remove the 'y' and add 'ies' to the noun. For example : hobbies, ladies, cities. It's a simple rule, and it always works. It also works when you are adding an 's' to verbs. For example, we write 'enjoys' and 'plays' by simply adding an 's', but we spell 'marries' and 'copies' with an 'ies' because there is a consonant before the 'y' in the base form of the verb.

Hobby --> Hobbies

Hobbies. Drop the Y and add ies.