"In the beautiful temple is a seer."
Translation:In templo pulchro est vates.
The "-um" vs "-o" per se is because it is 2nd declension neuter.
in + ablative is for static locations
in/ad + accusative is for motion toward
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English
Adjectives must agree in gender, number, and case with the nouns they modify, but they have their own declensions. Sometimes you get lucky and the adjective just happens to follow the same declension as the noun, but that is not a guarantee.
There is no direct object here. "Templo" is only the object of the preposition "in" and describes where our "vates" is. "Esse/to be" does not take objects because it is a verb of state, not a verb of action.
But yes, you can say "The seer is in the beautiful temple". That is the usual way to say it in English.
Prepositions in Latin take different cases, some can take multiple different cases each with differing meanings.
in with an ablative specifies a location, no movement to or from. in templo pulchro -> 'in the beautiful temple'
in with an accusative specifies movement to that location. in templum pulchrum -> 'into the beautiful temple'