177. The Perfect Stem is formed as follows.

a. The suffix v (u) is added to the verb stem,

vocā-v-ī, audī-v-ī

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.

carp-s-ī (CARP), 
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.

lēg-ī (lĕg-ō) 
ēm-ī (ĕm-ō) 
vīd-ī (vĭd-e-ō) 
fūg-ī (fŭg-i-ō) 
ēg-ī (ăg-ō)

e. Sometimes the perfect stem has the same formation that appears in the present tense.

vert-ī (vert-ō) 
solv-ī (solv-ō)

f. Sometimes the perfect is formed from a lost or imaginary stem.

petī-v-ī (as if from †peti-ō, †petī-re, PET)

Footnotes