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Ancient Greek verbs often alter the stem in the present system with a variety of markers. These markers are best understood as markers of the continuous and progressive aspects, rather than of the present tense.
For verbs with progressive markers, the present progressive system is not the best guide to the true stem, which is often more clearly manifested in the aorist or future tense forms. Note that none of these markers were productive in the Classical period, although many verbs had alternate forms with and without the markers even into the Hellenistic period and later. Many of these markers date back to Proto-Indo-European and have clear parallels in other Indo-European languages. However, some Ancient Greek grammars and textbooks list and discuss these markers to help students grappling with the confusing morphology of Ancient Greek verbs.