Why is the plural of 'nemesis' "nemeses" and not 'nemesii'?
and how does one pronounce "nemeses" different from "nemesis"?
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Nemesis is directly taken from Latin (which borrowed it from Greek). In Latin, almost all words ending in -is have -es as plural form. You will see the -ii plural form only for nouns which end in -ius.
You can hear the difference between the pronunciation here; nemesis rhymes on this and nemeses rhymes on these.