Both seem to mean “to kill” or “to slay” as far as I can see.
Have you looked either up in a dictionary?
https://logeion.uchicago.edu/ἀποκτείνω
https://logeion.uchicago.edu/κτείνω
Amongst other differences you will see that ἀποκτείνω is a “stronger form of κτείνω, kill, slay, Ep., Ion., and the prevailing form in Att. (cf. ἀποθνῄσκω): once in A. Ag. 1250, never in S., freq. in E., Hec. 1244, al.”
I did, but I can’t make too much use of them because of the abbreviations and frankly I don’t know how to read them. And more importantly, in my little experience, dictionaries cannot replace the knowledge and experience of someone who has seen a lot of examples and knows the nuances that cannot really be displayed in 2-3 words in dictionaries, but have to be described.
And I’m not sure what “a stronger form of” means.
Hi
You can find a list of the abbreviations used here https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Afrontmatter%3D0 and https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Afrontmatter%3D1
In the bit of the dictionary that I quoted Ep. means epic Ion. is ionic Att is attic. These are dialects. A. Ag 1250 is line 1250 of Aeschylus Agamemnon, never in S. = not found in Sophocles. freq. in E. =frequently in Euripides. Hec. =Hecuba of Euripides.
Learning to use a dictionary might be more useful to you rather than getting advice on usage. You will then be able to look particular lines up and see for yourself how they are used.
Looking at the entry for κτείνω you see that it is frequently in the poets but in prose ἀποκτείνω is more usual. You can also see that meaning 3. “to be fatal” is not one that appears for ἀποκτείνω. ἀποκτείνω has a judicial meaning condemn to death not cited in κτείνω.
“Stronger form” I take to mean as the prefix ἀπο acts as an intensifier. The Cambridge Greek lexicon which I recommend if you don’t have a good dictionary gives a meaning 3. of destroy someone (by banishment) as a hyperbolic way of describing banishment.
If you are a newcomer to Greek I wouldn’t get too bogged down in looking for nuances. There are more important things to learn.
What prompted you to ask this question?
Nothing special, just saw the two words in different textbooks, checked logeion, and then you know the rest.