The affix tumun comes after a verb when another verb meaning 'to wish' is in construction, provided that the agent of both the verbs is the same.,
Thus इच्छति, वष्टि or वाञ्छति भोक्तुम् 'he desires to eat'; कामयते भोक्तुम् 'he wishes to eat'; पिनाकपाणिं पतिम् आप्तुम् इच्छति 'desires to obtain for her husband the Pinaka-handed God Siva'.
But why do we say 'when the agent of the Infinitive and the verb are the same'? We can not say देवदतं भोक्तुम् इच्छति यज्ञदत्तः 'Yajnadatta wishes Devadatta to eat'; for here the agents of भुज् and इष् are not the same. We must say, भुञ्जानम् instead of भोक्तुम् ।
Why do not we use it here:- इच्छन् करोति 'wishing, he does'? Because the Infinitive is never found in such constructions. In other words, the Infinitive in तुम् is employed when both the verbs have the same agent.,