The verbs τίθημι, ἵ_ημι, δίδωμι form the singular active of the first aorist in -κ-α, thus, ἔθηκα, ἔδωκα, ἧκα. The forms of the second aorist ( cross756) are generally used in the dual and plural and in the other moods.
a. The form in κ rarely appears outside of the singular, chiefly in the 3 pl., as ἔδωκαν ( = ἔδοσαν), less frequently in the 1 and 2 pl., as ἐδώκαμεν, -ατε.
b. That κ was not a suffix but a part of an alternative root appears from a comparison of θηκ- in ἔθηκα and perf. τέθηκα with fēc- in fēci.
c. ἵστημι has ἔστησα
d. ἐθηκάμην is un-Attic; ἡκάμην (in comp.) is rare and probably found only in the indic.; ἐδωκάμην is very late.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].