When we conjugate the 3rd person singular of the present simple tense of verbs ending in -y sometimes we wonder why the English drop the final -y and add an -i plus -es and some other times they just add a -s.
This is my rule: get 2 vowels for a fly. I will show this by means of an example:
We conjugate the present simple tense in the affirmative form of to MARRY like this:
I marry, you marry, she marries, we marry, you marry, they marry.
MARR-Y only has a vowel (considering the semivowel -y a vowel itself). According to the rule we must get 2 vowels, therefore we drop the -y and add -i plus -es. The result is: marries. (You can count up to 2 vowels) Otherwise that would be marrys which is not real English.
Take now the present simple tense in the affirmative of to PLAY like this:
I play, you play, she plays, we play, you play, they play
PLAY-S has got 2 vowels so there must not be any dropping of vowels.