żeby
is a combination of the conjunction że (that) and the conditional particle by (roughly 'would', 'should' or subjunctive)
żeby is followed by an infinitive (impersonal) or a past stem (looks like third person singular of the past tense)
żebyś can only be followed by the past stem and only with second person singular (familiar) subject
żeby być = in order to be
żeby był = that he might be, that he be (subjunctive), that he should be
żebyś był = (same as above but with 'you' instead of 'he')