दुस्-

• दुः- (duḥ-) – before श्, ष्, स् (ś, ṣ, s) and rarely before क्, ख्, प्, फ् (k, kh, p, ph)
• दुर्- (dur-) – before vowels and voiced consonants
• दुश्- (duś-) – before च्, छ् (c, ch) and rarely before श् (ś)
• दुष्- (duṣ-) – before क्, ख्, प्, फ् (k, kh, p, ph) and rarely before ष् (ṣ); in older language also before त्, थ् (t, th) which become ट्, ठ् (ṭ, ṭh)
• दू- (dū-) – before र् (r) and sometimes before द्, ध्, न् (d, dh, n), which become ड्, ढ्, ण् (ḍ, ḍh, ṇ)

From Proto-Indo-Aryan *duš-, from Proto-Indo-Iranian *duš-, from Proto-Indo-European *dus-. Cognate with Ancient Greek δυσ- (dus-, “hard, difficult”), Old Armenian տ- (t-, a negative prefix), Avestan 𐬛𐬎𐬱-‎ (duš-), Gothic 𐍄𐌿𐌶- (tuz-); Old Irish do-.

दुस्- • (dus-)

1. a prefix to nouns and rarely to verbs or adverbs (Pāṇ. 2-1, 6 ; 2, 18 Vārtt. 2 Pat. ; iii, 3, 126, etc.) implying evil, bad, difficult, hard
2. badly, hardly ; slight, inferior, etc., often = English in- or un-.

• सु- (su-)

• Monier Williams (1899), “दुस्-”, in A Sanskrit–English Dictionary, […], new edition, Oxford: At the Clarendon Press, OCLC 458052227, page 488.