Breakdown of Post longa tago en la oficejo, ni preparas legoman supon kaj manĝas ĝin kune.
manĝi
to eat
la
the
ni
we
kaj
and
Questions & Answers about Post longa tago en la oficejo, ni preparas legoman supon kaj manĝas ĝin kune.
Why is it post longa tago and not post longan tagon?
Because post is a preposition, and prepositions in Esperanto normally take the basic form, not the accusative. So longa tago is correct here. The -n ending is mainly used for direct objects and for direction in some expressions, and neither applies after post in this sentence.
Why is there no la before longa tago?
Because the sentence means after a long day in a general or indefinite sense. If you said post la longa tago, it would sound like you were referring to one specific long day already known from the context.
Why do we say en la oficejo here? Does it mean in the office or at the office?
Literally, en means in, but Esperanto often uses en where English would naturally say at. So en la oficejo can be understood as in the office or more naturally at the office, depending on context.
Does en la oficejo describe the long day, or does it describe where we prepare the soup?
Most naturally, it describes la longa tago. So the idea is after a long day at the office. The comma helps show that Post longa tago en la oficejo is one opening time-setting phrase before the main action begins.