The principal parts of the Latin verb are the first person singular of the present indicative, the present infinitive, the first person singular of the perfect indicative, and the perfect passive participle.

184. Conjugation Stems.  From the principal parts we get three conjugation stems, from which are formed the entire conjugation.  We have already learned about the present stem\, which is found from the present infinitive (cf.  Sec. 126.a).  The other two stems are the perfect stem\ and the participial stem\.

185. The Perfect Stem.  The perfect stem of the verb is formed in various ways, but may always be found by dropping -i:  from the first person singular of the perfect, the third of the principal parts.  From the perfect stem are formed the following tenses: 

  THE PERFECT ACTIVE INDICATIVE
  THE PLUPERFECT ACTIVE INDICATIVE (ENGLISH PAST PERFECT)
  THE FUTURE PERFECT ACTIVE INDICATIVE

All these tenses express completed action in present, past, or future time respectively.

186. The Endings of the Perfect.  The perfect active indicative is inflected by adding the endings of the perfect to the perfect stem.  These endings are different from those found in any other tense, and are as follows: 

     SINGULAR PLURAL
  1. -i:, I 1. -imus, we
  2. -isti:, you 2. -istis, you
  3. -it, he, she, it 3. -e:runt or -e:re, they

187. Inflection of sum\ in the perfect, pluperfect, and future perfect indicative: 

               PRES.  INDIC.  PRES.  INFIN.  PERF.  INDIC. 
  PRIN.  PARTS sum esse fui: 

PERFECT STEM fu-

    PERFECT
  SINGULAR PLURAL
  fu’i:, I have been, I was fu’imus, we have been, we were
  fuis’ti:, fuis’tis, you have been, you were
    you have been, you were
  fu’it, he has been, he was fue:’runt or fue:’re,
                                   they have been, they were

PLUPERFECT (TENSE SIGN -era:-)
fu’eram, I had been          fuera:’mus, we had been
fu’era:s, you had been       fuera:’tis, you had been
fu’erat, he had been         fu’erant, they had been

    FUTURE PERFECT (TENSE SIGN -eri-)
  fu’ero:, I shall have been fue’rimus, we shall have been
  fu’eris, you will have been fue’ritis, you will have been
  fu’erit, he will have been fu’erint, they will have been

  1.  Note carefully the changing accent in the perfect.

  2.  Observe that the pluperfect may be formed by adding eram\, the
  imperfect of sum\, to the perfect stem.  The tense sign is -era:-.