In a mirror (di' esoptrou). By means of a mirror (esoptron, from optō, old word, in papyri). Ancient mirrors were of polished metal, not glass, those in Corinth being famous.
Darkly (en ainigmati). Literally, in an enigma. Old word from ainissomai, to express obscurely. This is true of all ancient mirrors. Here only in N.T., but often in lxx. “To see a friend’s face in a cheap mirror would be very different from looking at the friend” (Robertson and Plummer).
Face to face (prosōpon pros prosōpon). Note triple use of pros which means facing one as in Joh 1:1. Prosōpon is old word from pros and ops, eye, face.
Shall I know (epignōsomai). I shall fully (epi̇) know. Future middle indicative as ginōskō (I know) is present active and epegnōsthēn (I was fully known) is first aorist passive (all three voices).