Subjunctive.—Attic τιθῶ, etc., are derived by contraction from the forms of the weak stem to which the thematic vowel ώη has been added. Thus τιθέω, -έῃς, -έῃ, τιθέωμεν, -έητε, -έωσι; διδόω, -όῃς, -όῃ, διδόωμεν, -όητε, -όωσι. ἱστῶ is derived from ἱστέω. See cross746 b. Verbs in -νυ_μι regularly inflect the subjunctive like ω-verbs: δεικνύω, -ύῃς, -ύῃ.
a. Similarly the middle (passive) forms are derived from τιθέω-μαι τιθέη- (ς) αι, etc., διδόω-μαι διδόη- (ς) αι, ἱστέω-μαι ἱστέη- (ς) αι, etc. For the loss of ς in -σαι see cross465 a. -νυ_μι verbs inflect the mid. subj. like λύ_ωμαι.
c. Traces of -υ_ται in -νυ_μι verbs are very rare: ῥήγνυ_ται Hipponax 19; cp. διασκεδάννυ_ται
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].