A.T. Robertson Commentary 1 John 1:6

"If we say that we have fellowship with him and walk in the darkness, we lie, and do not the truth:" — 1 John 1:6 (ASV)

If we say (εαν ειπωμεν). Condition of third class with εαν and second aorist (ingressive, up and say) active subjunctive. Claiming fellowship with God (see verse 3) involves walking in the light with God (verse 5) and not in the darkness (σκοτος here, but σκοτια in Joh 1:5). See 2:11 also for εν τη σκοτια περιπατεω.

We lie (ψευδομεθα). Present middle indicative, plain Greek and plain English like that about the devil in Joh 8:44.

Do not the truth (ου ποιουμεν την αληθειαν). Negative statement of the positive ψευδομεθα as in Joh 8:44. See Joh 3:21 for "doing the truth," like Ne 9:33.