This is from Smyth’s Greek Grammar for Schools and Colleges:
Section 2106:
2106. Verbs of Knowing and Showing.—After verbs signifying to know, be ignorant of, learn (not learn of), remember, forget, show, appear, prove, acknowledge, and announce, the participle represents a dependent statement, each tense having the same force as the corresponding tense of the indicative or optative with ὅτι or ὡς, the present including also the imperfect, the perfect including also the pluperfect.
Such verbs are: οἶδα, γιγνώσκω, ἐπίσταμαι, ἐννοῶ, μανθάνω (2136), (οὐκ) ἀγνοῶ, μέμνημαι, ἐπιλανθάνομαι (2134), δηλῶ, (ἐπι) δείκνυ_μι, φαίνω, ἀποφαίνω, φαίνομαι (2143), ἔοικα (2089 c, 2133), (ἐξ-) ἐλέγχω, ὁμολογῶ (rarely), ἀγγέλλω, ποιῶ represent (2115).
Section 2139:
οἶδα and ἐπίσταμαι with part. in O. O. (2106) = know that something is; with inf. not in O. O. = know how to do something. Thus, ““ἐπιστάμενος νεῖν” knowing how to swim” X. A. 5.7.25. In poetry (very rarely in prose, except with ἐπίσταμαι in Hdt.) these verbs take also the inf. (in O. O.) in the meaning know or believe: ““ἐπιστάμεθα μή πώ ποτ᾽ αὐτὸν ψεῦδος λακεῖν” we know that he has never yet spoken falsehood” S. Ant. 1094.
Perhaps “οἶσθα ἡμᾶς σοι ἀεὶ βοηθόντας”
Regarding the contrafactual, εἰ + aor. ind. in protasis, aor. ind. + ἄν in the apodosis.