EXCEPTIONS OF गुण-सन्धि - 1

In the following cases, instead of the substitution of गुण vowels, वृद्धि vowels are substituted.

• When ऊह (formed from root वाह् - 1 आ. ) occurs immediately after अ/आ.
  E.g. प्रष्ठ+ऊह= प्रष्ठौहः ।   
• When ऊह , ऊढ or ऊढि (from root वह् -1 प.) occur immediately after the prefix प्र.
  E.g. प्रौहः , प्रौढः , प्रौढिः । (But प्रोढवान्, because ऊढवत् is not included here)       
• When अक्ष is compounded with ऊहिनी to form "अक्षौहिणी" - "an army consisting of 21870 chariots and elephants, 65610 horses and 109350 men". (Change of न् to ण् will be explained in detail later)
• When ईर, ईरिन्/ईरिणी (from verb-root ईर् - 1 प., 2 आ., 10 उ.) occur immediately after स्व.
  E.g. स्वैरः , स्वैरी, स्वैरिणी । 
• When ऋत (from root ऋ -1 प, 3 प.) occurs after a पद ending in अ/आ in an Instrumental तत्पुरुष समास.
  E.g. सुखार्तः ।( सुखेन ऋतः । )