Learn Russian: Разве он всерьёз бормотал эти слова? - Was he really muttering these words seriously?

Разве is a question particle used to express doubt, surprise, or disbelief. It usually translates to 'really?', 'is it true that?', or 'can it be that?'. Unlike a neutral question word, using разве here shows the speaker is surprised that he was muttering the words seriously and expects confirmation or clarification.

Could I use Неужели instead of Разве here?

Yes, but it changes the nuance slightly. Both express surprise, but неужели is stronger and often implies a higher degree of disbelief, shock, or emotion. Разве is a bit milder, often used when you are just seeking to clarify something that seems unlikely.

Why use the adverb всерьёз instead of серьёзно?

While both mean 'seriously', всерьёз is specifically used to mean 'in earnest' or 'not as a joke'. Серьёзно is a broader word that can mean 'seriously' in the sense of severe or thoughtful. Here, the speaker is asking if he meant the words or was just joking, making всерьёз the perfect fit.

Why is the imperfective бормотал used instead of its perfective counterpart пробормотал?