Origins
The Greek pure verbs can be assigned to several derivational types in the preceding Proto-Indo-European language:
Most of the verbs in -αω are derived from nominal ā-stems by forming the present tense with the suffix -i̯e-/-i̯o-: νικάω < *νῑκᾱ-i̯o- 'to win, prevail', from νίκα 'victory'; τιμάω < *τῑμᾱ-i̯o- 'to honor, revere', from τιμά 'honor'. A few verbs of this class underived verbs, e.g. δράω 'to do', σπάω 'to draw, pull'.
Verbs in -εω are derived from a range of nominal stems: φιλέω < *φιλε-i̯o- 'to love', from φίλος 'dear, beloved'; τελέω < *τελεσ-i̯o- 'to finish', from τέλος 'target, destination'; φωνέω < *φωνε-i̯o- 'to make a sound', from φονή 'sound'; μαρτυρέω < *μαρτυρε-i̯o- 'to testify', from μάρτυς 'witness'. Examples for underived verbs in this class are: ῥέω < *ῥεϝω 'to flow', ζέω < *ζεσω 'to boil'.
The verbs in -οω are mainly factitives derived from nominal o-stems: δηλόω 'to clarify, reveal', from δῆλος 'clear, obvious', δουλόω 'to enslave, conquer', from δοῦλος 'servant, slave'.
The verbs in -ιω are derived from nominal i-stems: μηνίω 'to be angry', from μῆνις 'anger, wrath'.
Among the verbs in -υω, there are underived ones, e.g. φύω 'to produce, issue', as well as some denominal verbs derived from u-stems, e.g. μεθύω 'to be drunk', from μέθυ 'wine'.
The verbs in -ευω are derived from nominal consonant stems: παιδεύω 'to educate', from παῖς 'child', δουλεύω 'to serve, to be a slave', from δοῦλος 'servant, slave'.
The verbs in -αιω are derived with the -i̯e-/-i̯o-suffix from roots ending in u̯: καίω < *καϝ-i̯o- 'to burn'; κλαίω < *κλαϝ-i̯o- 'to weep, wail'.