This is the 10031st most frequent Russian word.
архитектуру
Architecture.
The word 'архитектуру' is in accusative case, used as the direct object of the verb 'изучаем' to specify what is being studied.
Мы изучаем архитектуру старого города.
We are studying the architecture of the old town.
Here, the word 'архитектурой' is in instrumental case, used as part of a prepositional phrase indicating the object of admiration.
Он восхищается архитектурой современных зданий.
He admires the architecture of modern buildings.
In this sentence, 'архитектуру' is in the accusative case, referring to the specific architecture designed by the architect.