[818.] (1.) Some verbs have more than one form of the present stem: thus,
[819.] (a.) Verbs in -ere have rarely forms of verbs in -ēre in the present system: as, abnueō, nod no, abnuēbunt (Enn.), for abnuō, abnuent; congruēre, to agree (Ter.), for congruere. For verbs in -iō, -ere (or -ior, -ī), with forms of verbs in -īre (or -īrī), see [791]. Once pīnsībant (Enn.).
[820.] (b.) Some verbs in -āre have occasionally a present stem like verbs in -ere: as, lavis, washest, lavit, &c., for lavās, lavat, &c.; sonit, sounds, sonunt, for sonat, sonant. Others have occasionally a present stem like verbs in -ēre: as, dēnseō, thicken, dēnsērī, for dēnsō, dēnsārī.
[821.] (c.) Some verbs in -ēre have occasionally a present stem like verbs in -ere: as, fervit, boils, fervont, for fervet, fervent. See also fulgeō, oleō, scateō, strīdeō, tergeō, tueor in the dictionary. cieō, set a going, sometimes has a present stem in -īre, particularly in compounds: as, cīmus, ciunt, for ciēmus, cient.
[822.] (d.) Some verbs in -īre have occasionally a present stem like verbs in -ere: as, ēvenunt, turn out, for ēveniunt; ēvenat, ēvenant, for ēveniat, ēveniant, and advenat, pervenat, for adveniat, perveniat (Plaut.).
[823.] (2.) Some verbs have more than one form of the perfect stem: as,
eō, go, old īī ([765]), common iī, rarely īvī ([767]); pluit, it rains, pluit, sometimes plūvit. See also pangō, parcō, clepō, vollō or vellō, intellegō, pōnō, nectō, and adnectō, saliō and īnsiliō, applicō, explicō and implicō, dīmicō and necō in the dictionary. Some compound verbs have a form of the perfect which is different from that of the simple verb: as, canō, make music, cecinī, concinuī, occinuī; pungō, punch, pupugī, compunxī, expunxī; legō, pick up, lēgī, dīlēxī, intellēxī, neglēxī; emō, take, buy, ēmī (adēmī, exēmī), cōmpsī, dēmpsī, prōmpsī, sūmpsī.
[FORMATION OF STEMS.]
[VARIABLE VOWEL.]
[824.] The final vowel of a tense stem is said to be variable when it is -o- in some of the forms, and -u-, -e-, or -i- in others.