-θι is not rare in Hom., pres. δίδωθι δίδου, ὄρνυθι, aor. κλῦθι, perf. τέτλαθι. Aeolic has ἴστα_, φίλη. πίει, δέχοι, δίδοι (Pindar) are very rare.
b. -ς occurs in θές, ἕς, δός, σχές (and in the rare θίγες, πίεις). This -ς is not derived from -θι.
c. λῦς-ον aor. act. and λῦς-αι aor. mid. are obscure in origin.
a. 2 Sing.—-σο retains its ς in the (rare) perf. of all verbs and in the pres. of μι-verbs (λέλυσο, τίθεσο, ἵστασο). Elsewhere ς is dropped, as in λύ_ου from λύ_ε-σο, λιποῦ from λιπέ-σο, θοῦ from θέ-σο, οὗ from ἕ-σο, πρίω from πρία-σο, τι_μῶ from τι_μάε-σο.
N.—τίθου, ἵστω, δίδου are poetic or late.
3. 3 Pl.—For -ντων and -σθων we find -τωσαν and -σθωσαν in prose after Thucydides, in Euripides, and in inscriptions after 300 B.C. Thus, λυ_έτωσαν, λυ_σάτωσαν, λυ_έσθωσαν, λυ_σάσθωσαν, λυθήτωσαν, λιπέτωσαν, λιπέσθωσαν, φηνάσθωσαν, φανήτωσαν, τι_μάσθωσαν, φιλείσθωσαν, γεγράφθωσαν, πεπείσθωσαν, τιθέτωσαν, διδότωσαν, θέτωσαν, τιθέσθωσαν, θέσθωσαν, -ἕτωσαν, -ἕσθωσαν.
N.—ἔστων for ὄντων is rare. Attic inscriptions have (very rarely) -ντωσαν.
3. Doric has also -ντω, as in παρεχόντω; Aeolic -ντον, as φέροντον. Doric has -σθω (pl.) and -σθων.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].