what is the difference between "happen" and "happens"

which one of the following sentence are correct and why ?

1-I’ll stand by you whatever [color=red]happens.

2-I’ll stand by you whatever [color=red]happen.

please correct my sentence becuse I’m always get confused between these 2 words.

best wishes for you .

Ryan.

Whatever should be followed by a singular verb.

COLLINS COBUILD on CD-ROM

thank you very much Hmtony for your explanation . now I know that the verb “happens” always comes after the pronoun “whatever” . in other words , we use “happen” or “happens” depending on the place of word in the sentence .

best wishes for you .

Ryan

Whatever is a singular pronoun. When it is used as the subject of a sentence the verb must agree with it (third person singular).