This is the 1376th most frequent Russian word.
таджикистан
Tajikistan, a country in Central Asia.
Here, 'Таджикистан' is used as the subject of the sentence, stating what Tajikistan is.
Таджикистан — это страна в Центральной Азии.
Tajikistan is a country in Central Asia.
In this sentence, 'Таджикистана' is the genitive case form used in conjunction with the preposition 'из', indicating the origin of movement.
Мой друг недавно вернулся из Таджикистана.
My friend recently returned from Tajikistan.
Here, 'Таджикистан' is in the accusative case, which is used for the direct object of the verb 'посетить' (to visit).