177. The Perfect Stem is formed as follows.
a. The suffix v (u) is added to the verb stem,
or to the root.
son-u-ī (sonā-re, root SON)
mon-u-ī (monē-re, MON treated as a root)1
Note— In a few verbs the vowel of the root is transposed and lengthened.
strā-v-ī (sternō, STAR)
sprē-v-ī (spernō, SPAR)
b. The suffix -s- is added to the root.
tēx-ī (for tēg-s-ī, TEG)2
Note— The modifications of the present stem sometimes appear in the perfect.
fīnx-ī (FIG, present stem fingĕ-)
sānx-ī (SAC, present stem sancī-)
c. The root is reduplicated by prefixing the first consonant—generally with ĕ, sometimes with the root-vowel.
ce-cid-ī (cadō, CAD)
to-tond-ī (tondeō, TOND)
Note—In fid-ī (for †fe-fid-ī, find-ō scid-ī (for †sci-scid-ī, scindō), the reduplication has been lost, leaving merely the root.
d. The root vowel is lengthened, sometimes with vowel change.
e. Sometimes the perfect stem has the same formation that appears in the present tense.
f. Sometimes the perfect is formed from a lost or imaginary stem.
petī-v-ī (as if from †peti-ō, †petī-re, PET)