A.T. Robertson Commentary John 13:17

"If ye know these things, blessed are ye if ye do them." — John 13:17 (ASV)

If ye know (ε οιδατε). Condition of first class assumed as true, ε and present (οιδατε used as present) active indicative.

If ye do (εαν ποιητε). Third-class condition, εαν and present active subjunctive, assumed as possible, "if ye keep on doing." Both conditions with the one conclusion coming in between, "happy are ye." Just knowing does not bring happiness nor just occasional doing.