ein
en
a
Esperanto has no word for "a" or "an"
leave them out in Esperanto where you would normally use them in English. For "a" in the sense of "some particular individual"
use the words "iu" (some) or "unu" (one). Use "po" for a meaning like "three dollars a pound" (po tri dolaroj por unu funto)
un
un
un