First Book of Sanskrit - Lesson I - 02

i. In the first conjugation अ is added to the root
ii. Prior to that the final vowel and the penultimate short of roots take their गुण (guNa) substitute

The गुण (guNa) function is defined as follows:

guNa [इ, ई ] --> ए
guNa [उ,ऊ ] --> ओ
guNa [ऋ, ॠ ] --> अर्
guNa [ऌ ] --> अल्

Example
first conjugation[ भू ] => भ् + guNa[ऊ] + अ = भ् + ओ + अ = भव्