-τικός

From -σις (-sis, verbal noun suffix) or -τος (-tos, verbal adjective suffix) +‎ -ικός (-ikós, adjective suffix), occurring in some original cases and later used freely by metanalysis.

-τῐκός • (-tikós) m (feminine -τῐκή, neuter -τῐκόν); first/second declension

1. Added to verbal stems to form adjectives: relating to, suited to, skilled in, able to, -ive

   ‎ποιέω (poiéō, “to make”) + ‎-τικός (-tikós) → ‎ποιητικός (poiētikós, “creative”)

2. Added to other stems to form adjectives, particularly those ending in vowels

   ‎ἔξω (éxō, “outside”) + ‎-τικός (-tikós) → ‎ἐξωτικός (exōtikós, “foreign”)

   ‎ναυ-ς (nau-s, “ship”) + ‎-τικός (-tikós) → ‎ναυτικός (nautikós, “seafaring”)

Certain sound changes happen before the suffix, as before other derivational or inflectional suffixes beginning in τ (t). Labial stops β, π, φ (b, p, ph) become π (p), dental stops δ, θ, τ (d, th, t) become σ (s), velar stops γ, κ, χ (g, k, kh) become κ (k).

• ‎βλαβ- (blab-, “harm”) + ‎-τικός (-tikós) → ‎βλαβ-τικός → βλαπτικός (blab-tikós → blaptikós, “harmful”)
• ‎πείθω (peíthō, “persuade”) + ‎-τικός (-tikós) → ‎πειθ-τικός → πειστικός (peith-tikós → peistikós, “persuasive”)
• ‎πρᾱγ- (prāg-, “do”) + ‎-τικός (-tikós) → ‎πρᾱγ-τικός → πρᾱκτικός (prāg-tikós → prāktikós, “pertaining to action”)

Contracted verbs, which have stems ending in a vowel, use the long form of the vowel – this happens, for instance, in the example above: ποιέ-ω (poié-ō) → ποιη-τικός (poiē-tikós).

• Smyth, Herbert Weir (1920), “Part III: Formation of Words”, in A Greek grammar for colleges, Cambridge: American Book Company, § 858