Actually, in English the word order changes the meaning slightly.
1) "Marcus usually swims at night" infers we already know that Marcus has a hibit of swimming, and we are saying when he does it. Thus, it is answering the question when, where the "what" is already known.
2) But the sentence "At night, Marcus usually swims" answers the question "What does Marcus do at night?" Answer: he usually swims. Thus, it is answering the question what, where the "when" is already known.
I would like to know from someone more skilled in Latin than I am how you would distinguish between these two shades of meaning.