deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
Tenses are replaced with a flat ending unless some sUtra says that they must or may be replaced with a bent ending.
All verbs take bent endings when they do not show the doer.
Most of the verbs show the doer either take flat endings only, or bent endings only, or can take both with no change of meaning. Thsi has no logic and is quite annoying.
A very few roots like yaj यज् and pac पच् can take either flat or bent with a subtle difference of meaning (explained in middle voice). But that is the exception rather than the rule.