[gigogkggi]

Astu (अस्तु) is imperative, asti, (अस्ति) is what would be used.

[danans]

Either works depending on the context. "astu" is more like "let it be so". Still, there is no single generic affirmative particle.

[DFHippie]

I believe the direct cognate of this in Latin is "esto".