How do you know when to change a word to either have an 'ies' ending or 'y' ending?

The basic rule is for nouns ending in 'y' preceded by a consonant, the plural is formed by dripping the 'y' and 'ies'. For nouns ending in 'y' preceded by a vowel, the plural is formed by adding 's' only. Why this came about, I do not know. Perhaps a question for the category 'Word origins'.

Some examples for nouns that drop the Y and add IES for plurals are:

• babies
• daisies
• ladies
• stories
• dairies
• fairies
• parties
• countries

Some examples for nouns that do not drop the Y for plurals:

• alleys
• boys
• chimneys
• days
• essays
• joys
• keys
• monkeys