Learn Russian: Кто-то застучал в дверь. - Someone started knocking on the door.

The prefix за- is often added to verbs of sound or emotion to show that an action began suddenly or spontaneously. Here, adding it to стучать (to knock) creates the perfective verb застучать, which means to abruptly start knocking.

Could I just say Кто-то начал стучать instead?

Yes, начал стучать is grammatically correct and means 'started to knock.' However, using the prefixed verb застучал sounds much more natural and emphasizes that the knocking was sudden, unexpected, or broke the silence.

Why use кто-то (someone) here instead of кто-нибудь?

You use the suffix -то when the 'someone' definitely exists and the event actually happened, but you just do not know their identity. Since the knocking already happened in the past, a specific but unknown person did it. You would use кто-нибудь (anyone at all) in questions, requests, or hypothetical situations.

Why do we use the preposition в for knocking 'on' the door?

In Russian, the fixed phrase for knocking on a door or window uses the preposition в followed by the accusative case (here, в дверь). You can think of it as the force of the knock being directed 'into' the door, rather than just happening 'on' the surface (на).