To determine the root all suffixes must be removed from the stem until only that part remains which contains the fundamental idea.
a. Most roots are noun-roots or verb-roots; but originally a root was neither noun or verb ( cross193). Some roots are pronominal, and express direction or position. Greek has many words whose roots cannot be discovered. The form of a root in Greek is not necessarily that which Comparative Grammar shows was common to the cognate languages.
b. Since the origin of many words, even with the help of the cognate languages, is uncertain, we are often at a loss where to make the dividing line between root and suffix. Suffixes are often preceded by a vowel which may be regarded as a part of the suffix or as an expansion of the root (by some scholars regarded as a part of the root itself).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].