I wanted to pick a -μι verb to use as a paradigm to memorize for Homeric and koine, so I thought I would use ἀφίημι. I looked up the present-tense conjugation on U Chicago's morpho utility, and then noticed that the third person forms have π rather than φ, ἀπιεῖ. What is going on here? Is this a regular stem change, similar to the aorist of -ω verbs, or some other regular thing, or is this verb just irregular?
The forms with π are Ionic, which is a dialect that drops word-initial aspiration. It does not normally turn φ into π, but in this case the φ is a result of the π of the preposition ἀπό interacting with the initial aspiration of ἵημι.
The normal active present indicative third person singular in Attic is ἀφίησι(ν) or ἀφιεῖ.