Source: http://dcc.dickinson.edu/allen-greenough/Chapters/Chapter-179.html
Timestamp: 2019-04-24 07:58:07+00:00

Document:
a. The First Conjugation includes all verbs which add ā- to the root to form the present stem:1 as, amā-re; with a few whose root ends in a (†for, fā-rī; flō, flā-re; nō, nā-re; stō, stā-re).
The stem-vowel ā- is lost before -ō: as, amō = †amā-(y)ō; and in the present subjunctive it is changed to ē: as, amē-s, amē-mus.
The perfect stem regularly adds v, the supine stem t, to the present stem: as, amā-v-ī, amā-t-um. For exceptions, see § 209 . a.
b. The Second Conjugation includes all verbs which add ē- to the root to form the present stem: as, monē-re ; with a few whose root ends in ē; as, fle-ō, flē-re; ne-ō, nē-re; re-or, rē-rī (cf. § 176 . e).
In the present subjunctive ā is added to the verb-stem: as, mone-ā-s, mone-ā-mus (cf. § 168 . e).
A few verbs form the perfect stem by adding v (u), and the supine stem by adding t, to the present stem: as, dēlē-v-ī, dēlē-t-um. But most form the perfect stem by adding v (u) to the root, and the supine stem by adding t to a weaker form of the present stem, ending in ĭ: as, mon-u-ī, monĭ-t-um. For lists, see § 210.
c. The Third Conjugation includes all verbs (not irregular, see § 197) which add ĕ- to the root to form the present stem: as, tegĕre, capĕ-re; with a few whose root ends in e: as, se-rĕ-re for †se-se-re (reduplicated from SE, cf. sătum).
The stem-vowel ĕ is regularly lost before -ō, and becomes u2 before -nt and ĭ before the other endings of the indicative and imperative: as, teg-ō, tegi-t, tegu-nt; in the imperfect indicative it becomes ē : as, tegēbam, tegē-bās, etc.; in the future, ē : as, tegē-s (except in the first person singular, tega-m, tega-r); in the present subjunctive, ā : as, tegā-s.
Verbs in -iō lose the i before a consonant and also before ĭ, ī, and ĕ (except in the future, the participle, the gerund, and the gerundive). Thus, capi-at, capi-unt, capi-ēbat, capi-ēs, capi-et, capi-ent; but, cap-it (not †capi-it), cap-eret.
All varieties of perfect and supine stems are found in this conjugation. See lists, § 211. The perfect is not formed from the present stem, but from the root.
d. The Fourth Conjugation includes all verbs which add ī- to the root to form the present stem: as, audī-re.3 In these the perfect and supine stems regularly add v, t, to the verb-stem: as, audī-v-ī, audīt-um.4 Endings like those of the third conjugation are added in the third person plural of the present (indicative and imperative), in the imperfect and future indicative, and in the present subjunctive: as, audi-unt, audi-ēbat, audi-ētis, audi-at, the i being regularly short before a vowel.
e. The Present Imperative Active (second person singular) is the same as the present stem: as, amā, monē, tegĕ, audī. But verbs in -iō of the third conjugation omit i : as, capĕ (not †capie).
f. The tenses of completed action in the Active voice are all regularly formed by adding the tense-endings (given in § 166 ) to the perfect stem: as, amāv-ī, amāv-eram, amāv-erō, amāv-erim, amāv-issem, amāv-isse.
g. The tenses of completed action in the Passive voice are formed by adding to the perfect participle the corresponding tenses of continued action of the verb esse: as, perfect amātus sum; pluperfect amātus eram, etc.
The present stem is thus the verb-stem. For exceptions, see § 209 . a.
The gerundive varies between -endus and -undus .
A few are formed from noun-stems, as fīnī-re (from fīni-s ), and a few roots perhaps end in i ; but these are not distinguishable in form.
For exceptions, see § 212 . b.

References: § 209
 § 176
 § 168
 § 210
 § 197
 § 211
 § 166
 § 209
 § 212