a = indefinite article = not always necessary to include in Esperanto
one = very definite (about as definite as you can get) = always required when considered to be present in Esperanto.
Quite surprised not to see the English (UK) verion in here of just "Taso"! As there is no other kind of cup ;P
Which leads me to my question of would certain cultural habits end up in esperanto, even if somewhat against the original purpose of it? Or are these ferverently avoided by the community?