Learn Russian: Внезапно птица запела, и мы замолчали. - Suddenly the bird started singing, and we fell silent.

The prefix за- shows the sudden, spontaneous beginning of an action. While петь means "to sing," the perfective verb запеть means "to suddenly start singing" or "to burst into song." It pairs perfectly here with внезапно (suddenly).

Does the за- in замолчали do the exact same thing as in запела?

Yes! The imperfective verb молчать means "to be silent" or "to keep quiet." Adding за- indicates the sudden start of that state. "Starting to be silent" translates naturally into English as "falling silent" or "quieting down."

Why are запела and замолчали in the perfective aspect if singing and being silent are usually ongoing actions?

Because the за- prefix shifts the focus away from the ongoing action and onto the exact, single moment that the action began. They didn't just sing or stay silent in general; they completed the specific action of starting those states, which makes it perfective.

Why is there a comma before и (and) in this sentence?

In Russian grammar, a comma is required before и when it connects two complete, independent clauses (parts of a sentence that each have their own subject and verb). Since we have птица запела (the bird started singing) and мы замолчали (we fell silent), the comma is mandatory.