Learn Croatian: Kapetan stoji uz prozor. - The captain is standing next to the window.

The preposition uz always takes the accusative case, even when describing a stationary location. Unlike prepositions like u or na which change cases depending on whether there is motion, uz sticks to the accusative for physical alignment.

Does uz prozor mean the exact same thing as pored prozora?

They are very similar, but uz specifically implies being closely aligned with or right up against something, like standing alongside the glass. Pored simply means next to or beside.

Why doesn't the word prozor change its ending for the accusative case?

Because prozor is a masculine inanimate noun (an object, not a living being). For inanimate masculine nouns, the accusative form looks exactly like the nominative dictionary form. If it were a living thing, you would see an -a added, like uz brata.

Is stoji the present tense of stajati?

Yes, stoji is the third-person singular present tense of stajati (to stand). This verb is slightly irregular because the -ja- from the infinitive disappears in the present tense forms.