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"?

Solution 1:

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.

Solution 2:

Monies is one of those words where an error crept in and is now accepted as an exception. The plural of money (and there IS a plural in legal and accounting) should be, and is still acceptable as, moneys, and then there isn't an exception.

The general rule - if the Y is preceded by a consonant, then it is replaced by ies. If the Y is preceded by a vowel, add s.

This also works for verbs (I carry - he carries, I play, he plays). Nice and consistent.