For if I preach (εαν γαρ ευαγγελιζωμα). Third class condition, supposable case. Same construction in verse 1 Corinthians 9:16 (εαν μη).

For necessity is laid upon me (αναγκη γαρ μο επικειτα). Old verb, lies upon me (dative case μο). Jesus had called him (Acts 9:6; Acts 9:15; Galatians 1:15; Romans 1:14). He could do no other and deserves no credit for doing it.

Woe is me (ουα γαρ μο). Explaining the αναγκη (necessity). Paul had to heed the call of Christ that he had heard. He had a real call to the ministry. Would that this were the case with every modern preacher.