Learn Finnish: En pyydystänyt hiirtä, vaan se pääsi karkuun. - I did not catch the mouse, but it got away.

In Finnish, the object of a negative sentence must always be in the partitive case. Because the first clause starts with the negative verb En pyydystänyt (I didn't catch), the base word hiiri becomes hiirtä.

Why does this sentence use vaan instead of mutta for "but"?

Both words mean "but", but vaan is specifically used after a negative statement to introduce a direct contrast or correction, often translating to "but rather". Since you didn't catch the mouse, vaan introduces the contrasting reality of what happened instead.

How does the phrase pääsi karkuun translate to "got away"?

This common idiom uses the verb päästä (to manage to get to, to reach) combined with karkuun. Karkuun is the illative case (the "into" form) of the noun karku (flight or escape). Literally, it means the mouse "managed to get into escape".

Could I just use the verb karata (to escape) instead of päästä karkuun?

Yes, you could technically say vaan se karkasi. However, päästä karkuun emphasizes the successful result of getting away despite someone's active attempt to catch it, making it a much more natural fit for the context of this sentence.