I planted (egō ephuteusa). First aorist active indicative of old verb phuteuō. This Paul did as Luke tells us in Acts 18:1-18.
Apollos watered (Apollōs epotisen). Apollos irrigated the church there as is seen in Acts 18:24-19:1. Another aorist tense as in 1Co 3:2.
But God gave the increase (alla ho theos ēuxanen). Imperfect tense here (active indicative) for the continuous blessing of God both on the work of Paul and Apollos, Corinthians-labourers with God in God’s field (1Co 3:9). Reports of revivals sometimes give the glory to the evangelist or to both evangelist and pastor. Paul gives it all to God. He and Apollos cooperated as successive pastors.