Primary Endings versus Secondary

Compare: primary endings for present tense
Compare: secondary endings for imperfect (or 2nd aorist, chapter 11)

primary and secondary Watch out for the differences!

Present Tense    Imperfect Tense
stem + primary endings augmented stem + secondary endings
  am sitting   καθίζ ω   was sitting   ἐκάθιζ ον
you    are sitting   καθίζ εις you   were sitting ἐκάθιζ ες
He   is sitting καθίζ ει he   was sitting ἐκάθιζ ε (ν)
We   are sitting καθίζ ομεν we   were sitting ἐκαθίζ ομεν
You   are sitting καθίζ ετε you   were sitting ἐκαθίζ ετε
They    are sitting καθίζ ουσιν they   were sitting ἐκάθιζ ον

Note the movable ν in the 3rd person singular used at the end of a clause, or before words starting with a vowel.