The affix a comes after a verb which has a prosodially heavy vowel and ends in a consonant when the word to be formed is feminine.,
This debars क्तिन् । Thus कुण्डा 'an adulteress'; हुण्डा 'a tigress'; ईहा 'effort'; उहा 'supplying an ellipsis'.
Why do we say 'which has a prosodially heavy vowel'? Observe भक्ति devotion'. Why do we say 'which ends in a consonant'? Observe नीति 'polity'.,