The preposition post, like many prepositions, takes the accusative, never the ablative.
When talking about a specific time, with a word like annus, you might see something like multis post annis, but in such a case post is working as an adverb (i.e. “afterwards”) and still is not taking an ablative object.
Also, in your translation, videbant should be translated in the third person, not first person.
Could someone explain the difference between “te” and “tu”?
tu is the nominative second person singular personal pronoun. It corresponds to the archaic English “thou” or “you” (singular nominative).
te is the accusative and ablative second person singular personal pronoun. It corresponds to the archaic English “thee” or “you” (singular accusative/ablative).