Participles of the active voice (except the perfect, cross309), and the aorist passive participle have stems in ντ. The masculine and neuter follow the third declension, the feminine follows the first declension.
a. Most stems in οντ make the nom. sing. masc. without ς, like γέρων ( cross243). But stems in οντ in the present and second aorist of μι-verbs (διδούς, δούς), and all stems in αντ, εντ, υντ, add ς, lose ντ ( cross100), and lengthen the preceding vowel (-ους, -α_ς, -εις, -υ_ς, cross37). In like manner the dat. pl. is formed: -οντ-σι -ουσι, etc.
N.—The stem of participles in -ων, -οντος was originally ωντ. γέρων was orignally a participle.
b. The nominative neuter of all participles drops final τ of the stem ( cross133).
c. The perfect active participle (stem οτ) has -ως in the masculine, -ος in the neuter. -ως and -ος are for -ϝωτ-ς, -ϝοτ-ς.
d. The feminine singular is made by adding yα to the stem. Thus, λύ_ουσα (λυ_οντ-yα), οὗσα (ὀντ-yα), ἱστᾶσα (ἱσταντ-yα), τιθεῖσα (τιθεντ-yα). The perfect adds -υ (ς) -yα, as in εἰδ-υῖα.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].