Learn Finnish: Saalis havaitsee kotkan, mikä pelastaa sen täpärästi. - The prey detects the eagle, which saves it narrowly.

In Finnish, joka refers back to the immediately preceding noun. If you used joka here, it would refer to the eagle (kotkan), which would mean the eagle saved the prey! Because you are referring to the entire preceding clause (the fact that the prey spotted the eagle), you must use mikä.

Why are the objects kotkan and sen in the genitive case rather than the partitive?

Both havaita (to detect) and pelastaa (to save) express completed, result-oriented actions here, rather than ongoing processes. Therefore, their singular objects take the total object case, which uses the genitive ending -n in standard affirmative sentences.

Why does the sentence use sen (it) as the object instead of hänet?

In standard written Finnish, hän and its object form hänet are strictly reserved for humans. Because the prey (saalis) is an animal, the sentence uses the pronoun se, which takes the form sen as a total object.

How is the word täpärästi formed?

It is formed by taking the adjective täpärä (close or narrow, as in a "close call") and adding the adverb suffix -sti. This is the standard way to turn an adjective into an adverb in Finnish, much like adding "-ly" in English.