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 1. John 1:3) involves walking in the light with God (verse 1. John 1:5) and not in the darkness (σκοτος here, but σκοτια in John 1:5). See 1. John 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 John 8:44.

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