178. The Supine Stem may be found by dropping -um from the Supine. It is formed by adding t (or, by a phonetic change, s):
a. To the Present stem.
b. To the root, with or without ĭ.
cap-t-um (capiō, CAP)
moni-t-um (moneō, MON used as root)
cās-um (for †cad-t-um, CAD)
Note 1— By phonetic change dt and tt become s (dēfēnsum, versum for †dē-fendt-um, †vert-t-um); bt becomes pt (scrīp-t-um for †scrīb-t-um); gt becomes ct (rēc-t-um for †reg-t-um).1
Note 2— The modifications of the Present stem sometimes appear in the supine.
tīnc-t-um (tingō, TIG)
tēn-s-um for †tend-t-um (ten-d-ō, TEN)
Note 3— The Supine is sometimes from a lost or imaginary verb stem.
petī-t-um (as if from †peti-ō, †petī-re, PET)
Note 4— A few verbs form the supine stem in s after the analogy of verbs in d and t.
fal-s-um (fallō), pul-s-um (pellō)