For when one saith (οταν γαρ λεγη τις). Indefinite temporal clause with the present subjunctive of repetition (Robertson, Grammar, p. 972). Each instance is a case in point and proof abundant of the strife.

Of Paul (Παυλου). Predicate genitive, belong to Paul, on Paul's side.

Of Apollos (Απολλω). Same genitive, but the form is the so-called Attic second declension. See the nominative Απολλως in verse 1. Corinthians 3:5.

Men (ανθρωπο). Just mere human creatures (ανθρωπο, generic term for mankind), in the flesh (σαρκινο), acting like the flesh (σαρκικο), not πνευματικο, as if still ψυχικο. It was a home-thrust. Paul would not even defend his own partisans.