Learn Croatian: Stavlja pismo u poštanski sandučić. - He puts the letter in the postal mailbox.

Because the action is currently in progress (he is putting it right now) or happens habitually. If we used the perfective stavi, it would typically act as a future tense (he will put) or be used in a dependent clause, because perfective verbs cannot express an ongoing present action.

Why is u poštanski sandučić in the accusative case here, rather than the locative case?

The preposition u takes the accusative case when it indicates motion toward a destination. Since he is actively moving the letter into the mailbox, we use the accusative. If the letter were already resting inside the mailbox with no motion involved, we would use the locative case instead: u poštanskom sandučiću.

Since poštanski sandučić is in the accusative case, why does it look exactly like the nominative dictionary form?

Sandučić is a masculine noun. In Croatian, when masculine nouns represent inanimate objects (non-living things), their accusative form is identical to their nominative form. The adjective poštanski follows this exact same rule.

What is the root of the noun sandučić?

It is a diminutive form of sanduk, which means a chest, trunk, or large box. By adding the diminutive suffix -ić, it literally translates to a little box, which is the standard Croatian word for a mailbox.