-μι Verbs

Active Present Participles

As with all nouns (present participles count as nouns, whereas middle/passive participles are considered adjectives in Greek grammar) the stem is found depriving the genitive form of its 3rd declension genitive ending -ος.
To this stem is added -ς for masculine
To this stem is added -σα for feminine
To this stem is added nothing for neuter.

But now we've run into a problem: there is not such thing as words ending in -τ.
Cf τὸ ὄνομα, τοῦ ὀνόματ-ος, stem ὄνοματ: end -τ was dropped, purely and simply.

Next Greek idiosyncracy: the combination -ντ is dropped before σ and the preceding vowel is lengthened according to the following pattern:
ο lengthens to ου
ε lengthens to ει
α lengthens to ᾱ

So now we can correct the above table:

I said, the stem vowel is short, so we have:

  Note  

The masculine participles ἱστάς (as well as στάς aorist participle) and δεικνύς should have a macron indicating their length, but Unicode fonts don't like a combination macron+accent, so just imagine it's there, even if invisible like the Cheshire cat.

The aorist active participles of -μι verbs δίδωμι, τίθημι, ἵεμι and ἵστημι (intransitive only, i.e. stopping oneself) and compounds are the same as the present participle, minus the stem reduplication/extension of course: