Learn Russian: Строгий таможенник простоял там весь день. - The strict customs officer stood there all day.

Usually, yes. However, adding the prefix про- to a verb of state or continuous action (like стоять) creates a perfective verb meaning 'to spend a specific amount of time doing an action'. It packages the entire block of time (весь день) into a single, completed event.

Does using простоял change the tone of the sentence compared to just using the imperfective стоял?

Yes. While стоял just states the factual duration, using the про- perfective emphasizes that the time spent felt significant, whole, or perhaps tiring. It highlights that he actually stood there for that entire stretch of time.

Why is весь день in the accusative case if there is no direct object or preposition?

To express the exact duration of an action, Russian uses the accusative case without any preposition. Because день is a masculine inanimate noun, its accusative form is identical to its nominative form.

What is the structure of the word таможенник?

It is derived from the noun таможня (customs). It uses the -ник suffix, which is very commonly added to noun or adjective stems to denote a person's profession or role, similar to работник (worker).