The 3rd person singular is usually formed by adding s. But there are a few exceptions to the rule:
The verbs can, may, might, must remain the same in all forms. So don't add s.
example: he can, she may, it must
Verbs ending in o or a sibilant (ch, sh, s, x) add es instead of s.
example: do - he does, wash - she washes
A final y after a consonant becomes ie before s.
example: worry - he worries
But: A final y after a vowel (a, e, i, o, u) is not modified.
example: play - he plays