reduplication

Reduplication is the unique prefix given to perfect tense verbs in Greek.  If a verb begins with a consonant, that consonant is reduplicated and the two consonants are separated by an epsilon.

λυω (present tense) becomes λελυκα (perfect tense).

ποιεω (present tense) becomes πεποιηκεν (perfect tense).

If the consonant that was reduplicated is φ, χ, or θ then the reduplicated consonant will change to π, κ, or τ respectively. cf. BBG 25.8.

φανεροω (present tense) becomes πεφανερωκα (perfect tense)

τιθημι (present tense) becomes τέθειται (perfect tense).

Verbs beginning with a vowel or dipthong do this in an irregular way called “vocalic reduplication”.  Basically, vocalic reduplication is identical in form to the augment in the imperfect and aorist (BBG 25.8).

αγαπαω becomes ηγαπηκα

εγγιζω becomes ηγγικεν