Peculiarities

181. In tenses formed upon the Perfect Stem, v between two vowels is often lost and contraction takes place.

a. Perfects in -āvī, -ēvī, -ōvī, often contract the two vowels into ā, ē, ō, respectively.

amāsse for amāvisse
amārim for amāverim
amāssem for amāvissem
cōnsuērat for cōnsuēverat
flēstis for flēvistis
nōsse for nōvisse

So, also, in Perfects in -vī, where the v is a part of the Present stem.

commōrat for commōverat

Note— The 1st person of the Perfect Indicative (as, amāvī) is never contracted, the 3rd very rarely.

b. Perfects in -īvī regularly omit v, but rarely contract the vowels except before st and ss, and very rarely in the 3rd person Perfect.

audieram for audīveram
audīsse for audīvisse
audīstī for audīvistī
abiit for abīvit
abiērunt for abīvērunt

Note 1— The forms sīris, sīrit, sīrītis, sīrint, for sīveris etc. (from sīverō or sīverim), are archaic.

Note 2— In many forms from the Perfect stem -is-, -iss-, -sis-, are lost in like manner, when s would be repeated if they were retained.

dīxtī for dīxistī (x = cs)
trāxe for trāxisse
ēvāstī for ēvāsistī
vīxet for vīxisset
ērēpsēmus for ērēpsissēmus
dēcēsse for dēcessisse

These forms belong to archaic and colloquial usage.

182. Four verbs—dīcō, dūcō, faciō, ferō—with their compounds, drop the vowel termination of the Imperative, making dīc, dūc, făc, fĕr; but compounds in -ficiō retain it (cōnfice).

Note— The Imperative forms dīce, dūce, face (never fere), occur in early Latin.

a. For the Imperative of sciō, the Future form scītō is always used in the singular, and scītōte usually in the plural.

183. The following ancient forms are found chiefly in poetry:

1. In the 4th conjugation, -ībam, -ībō, for -iēbam, -iam (future). These forms are regular in eō go (§ 203).
2. In the Present Subjunctive, -im (as in duim, perduim is retained in religious formulas and often in comedy. This form is regular in sum and volō and their compounds (§ 170 and § 199).
3. In the Perfect Subjunctive and Future Perfect Indicative, -sim, -sō.
   

   faxim, faxō
   iussō
   recēpsō (= fēcerim etc.)
   ausim (= ausus sim)

4. In the Passive Infinitive, -ier.
   

   vocārier for vocārī
   agier for agī

5. A form in -āssō, -āssere is found used as a Future Perfect.
   

   amāssis from amō
   levāssō from levō
   impetrāssere from impetrō
   iūdicāssit from iūdicō

   Cf. § 263.2.b, Note