I. Present Stem, from which are formed—­
1.  Present, Imperfect, and Future Indicative,
2.  Present and Imperfect Subjunctive,
3.  The Imperative,
4.  The Present Infinitive,
— (Active and Passive.)
5.  The Present Active Participle, the Gerund, and Gerundive. 
II.  Perfect Stem, from which are formed—­
1.  Perfect, Pluperfect, and Future Perfect Indicative,
2.  Perfect and Pluperfect Subjunctive,
3.  Perfect Infinitive,
— (Active.)
III.  Participial Stem, from which are formed—­
1.  Perfect Participle,
2.  Perfect, Pluperfect, and Future Perfect Indicative,
3.  Perfect and Pluperfect Subjunctive,
4.  Perfect Infinitive,
— (Passive.)

Apparently from the same stem, though really of different origin, are the Supine, the Future Active Participle, the Future Infinitive Active and Passive.

THE FOUR CONJUGATIONS.

98.  There are in Latin four regular Conjugations, distinguished from each other by the vowel of the termination of the Present Infinitive Active, as follows:—­

INFINITIVE   DISTINGUISHING
CONJUGATION.   TERMINATION.       VOWEL. 
I.      -are            a
II.      -ere            e
III.      -ere            e
IV.      -ire            i

99.  PRINCIPAL PARTS.  The Present Indicative, Present Infinitive, Perfect Indicative, and the Perfect Participle[28] constitute the Principal Parts of a Latin verb,—­so called because they contain the different stems, from which the full conjugation of the verb may be derived.

* * * * *

CONJUGATION OF SUM.

100.  The irregular verb sum is so important for the conjugation of all other verbs that its inflection is given at the outset.

PRINCIPAL PARTS. 
PRES.  IND.         PRES.  INF.         PERF.  IND.      FUT.  PARTIC.[29]
sum               esse              fui             futurus

INDICATIVE MOOD.

PRESENT TENSE. 
SINGULAR.                              PLURAL.
sum, I am,                         sumus, we are,
es, thou art,                      estis, you are,
est, he is;                        sunt, they are.

IMPERFECT.
eram, I was,                       eramus, we were,
eras, thou wast,                   eratis, you were,
erat, he was;                      erant, they were.

FUTURE.
ero, I shall be,                   erimus, we shall be,
eris, thou wilt be,                eritis, you will be,
erit, he will be;                  erunt, they will be.

                                PERFECT.

fui, I have been, I was, fuimus, we have been, we were,
fuisti, thou hast been, thou fuistis, you have been_, you
wast, were_,
fuit, he has been, he was; fuerunt, fuere,

                                                                          they have been, they were.