Question

why is it correct to use discuss rather than discusses in some sentences
Writeacher
It depends on the subject. <b>If the subject is 3rd person, singular (noun or pronoun), then the present tense verb form is "discusses."</b> If the subject is anything else, singular or plural, noun or pronoun, then the verb form is "discuss."

I discuss...
you discuss ...
he, she, it (or any singular noun) discusses...

we discuss...
you (all) discuss...
they discuss...

http://grammar.ccc.commnet.edu/grammar/sv_agr.htm
In all the explanations and examples here, notice that all the<b> present tense, 3rd person singular verbs have the -s ending</b>; all others don't.