Karnatakabhashabhushana 7.17

[The suffixes] ntu and ndu [occur respectively] in the sense of manner (prakāra) and of time (kāla).

After these, namely adu, idu, and ēn, these two suffixes ntu and ndu occur in respective order in the sense of manner and in the sense of time.

In the sense of manner:

antu [‘in that manner’],

intu [‘in this manner’],

entu [‘in what manner?’].

In the sense of time:

andu [‘at that time’],

indu [‘at this time’],

endu [‘at what time?’].