Learn Polish: Myślę, że on trafił w samo sedno. - I think that he hit right at the core.

You could absolutely just say w sedno (into the core). Adding samo acts as an intensifier meaning 'the very' or 'exactly'. Here, w samo sedno translates to 'right at the core' or 'dead center', emphasizing perfect accuracy.

Why does the preposition w take the accusative case (samo sedno) here instead of the locative?

The preposition w only takes the locative case when describing a static location. Because the verb trafić (to hit) involves a directed action or motion toward a target, w takes the accusative case to show movement into that target.

Is trafić w samo sedno a set phrase in Polish?

Yes, it is a very common idiom that works exactly like the English phrase 'to hit the nail on the head'. You will hear this frequently in conversation when someone makes a perfectly accurate point or correctly identifies the crux of a problem.

How do I know samo means 'the very' here and not 'the same' or 'alone'?

To mean 'the same', Polish typically pairs it with a demonstrative pronoun, so you would see w to samo sedno. To mean 'alone' or 'by itself', the word usually follows the noun. When sam, sama, or samo is placed directly before a noun without a demonstrative, it almost always acts as this intensifier.