The affix ām is used when liṭ (Perfect Tense) follows after a root beginning with a letter of ic pratyahāra and having a heavy vowel, excepting the root ṛccha 'to go'.,
Thus ईह् 'to try', ईहाञ्चक्रे 'he tried'; so also ऊहाञ्चक्रे 'he guessed', एधाञ्चक्रे 'he grieved'.
But ऋच्छ् forms आनर्च्छ 'he failed or went'. This applies to monosyllabic roots which begin with any vowel prosodially long, except अ or आ. The verb ऊर्ण 'to cover' is, however, an exception; its perfect is ऊर्णुनाव 'he covered'.
Why do we say 'beginning with इच् or any vowel except अ or आ' ? Observe अर्द, Perfect आनर्द 7.4.71. Why do we say 'heavy vowel' ? Observe उख्, Perf. उवोख 6.4.78 ऊखतुः, ऊखुः. Why do we say 'vowel' ? Observe कुर्द, Perf. चुकुर्दे; षूद्, Perf. सुषूदे.,