The affixes tumun and ṇvul are placed after a verbal root with the force of the future when the word in construction therewith is another verb, denoting an action performed for the sake of the (future) action.,
As भोक्तुम् व्रजति 'he goes to eat'; भोजको व्रजति 'he goes to eat'. Here the upapada or the verb in construction is व्रजति; it denotes an action performed for the sake of the future action (क्रियार्था), namely, eating.
Why do we say 'denoting an action'? The upapada must be an action and not a substantive; as भिक्षिस्ये इति अस्य जटाः 'matted hair is for the purpose that he should beg.' Here the affixes are not employed.
Why do we say 'performed for the sake of another action'? Observe धावतस्ते पतिष्यति दण्डः 'running, thy stick will fall'.
Now, it might be asked why ण्वुल् is taught a second time, when it was already taught generally by rule 3.1.133; and that being a common rule, would have applied to this subject also? If you reply that the future affix लृट् would have prevented its application, in conditional sentences like the above, we say that the rule of 'non-uniform affixes applying simultaneously' given in 3.1.94 will step in and will conduce the application of ण्वुल् in spite of लृट्.
Vart:- The repetition of the affix ण्वुल् (ordained to come after a verbal root, when the word in construction therewith is another verb denoting an action performed for the sake of the future action,) is for the sake of prohibiting the application of the affixes like तृच् &c. in denoting the above sense; that is to say, the rule of वाऽसरूप 3.1.94 does not apply here and we cannot optionally employ any other affix, with this gerundial sense. Thus we cannot say कर्त्ता व्रजति 'he goes to do'; but we must say कर्तुम् instead.
These affixes form the gerund of the infinitive of purpose.,