Learn Croatian: Plovili smo danima po moru. - We sailed for days on the sea.

In Croatian, a noun in the bare Instrumental plural (without a preposition) is used to express continuous, uninterrupted duration. Words like danima (for days), satima (for hours), or godinama (for years) are the standard way to express this. You shouldn't use za here, as that translates "for" in the sense of a deadline, a future goal, or the time it takes to complete a specific task.

Why is the verb plovili (imperfective) required here?

Because the action went on continuously "for days" (danima), you must use an imperfective verb. Perfective verbs focus on the completion of an action, so they naturally clash with time expressions that emphasize an ongoing, uninterrupted duration.

Why use po moru instead of na moru to say "on the sea"?

While na moru simply means "at sea" or "on the sea" (indicating a static location), using po with the Locative case specifically conveys movement around, over, or across a surface. Since the subject is actively sailing, po moru perfectly captures that continuous movement across the water's surface.

If I want to emphasize that it lasted "for days," can I put danima at the very beginning of the sentence?