Learn Russian: Почему вы внезапно замолчали?

The base verb молчать simply means 'to be silent.' By adding the prefix за-, the verb takes on an ingressive meaning, focusing on the sudden, spontaneous beginning of an action. Instead of 'were silent,' замолчали means 'started being silent' or 'fell silent'.

Since замолчали already implies a sudden start, isn't the word внезапно (suddenly) repetitive here?

It might feel slightly repetitive, but it is extremely natural in Russian! Pairing adverbs like внезапно or вдруг (suddenly) with за- prefixed verbs is very common. It just adds extra emphasis to how abruptly the action started.

Are молчать and замолчать a standard imperfective/perfective pair?

Not exactly. молчать (imperfective) describes the ongoing state of being silent, while замолчать (perfective) focuses on the start of that state. If you need an imperfective verb for the process of someone's voice trailing off or quieting down before they fall completely silent, Russian uses a different form: замолкать.

Could I use this verb to tell someone to 'be quiet' or 'shut up'?

Yes! The imperative forms Замолчи! (informal singular) and Замолчите! (formal/plural) are standard ways to say 'Be quiet!' or 'Shut up!'. Because it is a perfective verb, it functions as a strong command to complete the action of falling silent right now.