deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
The "thumb" part of the rule is this —
(A)vowel-ender roots are usually /aniT, but U RR a ऊ ॠ अ-enders are /seT
(B)consonant-ender roots are usually /seT.
Now the exceptions.
The roots that end in consonants but are /seT are —
(1) All that end in U ऊ RR ॠ a अ, and
(2) vRG वृङ् vRJ वृञ् zvi श्वि /zIG DIG डीङ् ruG रुङ् snu स्नु kSu क्षु yu यु nu नु kSNu क्ष्णु UrNUJ ऊर्णूञ्.
For the roots that end in consonants but are /aniT, see list of aniT roots.