grammatical number - When should a singular word ending in "y" end in "ies" plurally?

Words like "sky" and "money" have "ies" as a plural suffix (i.e. "skies" and "monies") but other words like "monkey" and "Emmy" do not ("monkeys" and "Emmys"). Is there a rule dictating the use of "ies"?

Answer

It's determined by the letter before the y:

monkey: vowel + y => monkeys
sky: consonant + y => skies

Exception: proper nouns like "Emmy" sometimes form the plural by adding "s".

Monies I don't know about, so hopefully someone else can fill in that detail.