Suffix for noun following a prepositions and a singular noun
This is mentioned in Tips for Basics 2
The noun following a preposition normally takes a simple -o ending (-oj in the plural)
Isn't all nouns end in -o or -oj(plural)
Nouns not following a proposition may end in -on or -ojn (plural) if they are to be treated as direct objects; as in "mi legas libron". Maybe that's what they meant.
When following a preposition where there isn't motion, the noun (and any associated adjectives) are in the nominative case. (-o/-oj). For example:
La kato ĉasis la muson en la domo (the cat chased the mouse in [inside of] the house)
Since all the activity happens in the house, you don't use -n. (Even though chasing is motion, it isn't part of the preposition, so it doesn't count for this rule.)
When following a preposition where there is motion, then you would use the accusative case (-on/-ojn):
La kato ĉasis la muson en la domon (the cat chased the mouse into the house).
This time, in the preposition, there is movement from outside the house into the house. So we add the -n to make the distinction.