Learn Finnish: Nuori poika laskeutuu hypystä huolellisimmin. - The young boy lands from the jump the most carefully.

The base adjective is huolellinen (careful), which makes the basic adverb huolellisesti (carefully). To make the superlative adverb meaning 'the most carefully', you take the plural stem of the adjective, which is huolellisi-, and add the superlative adverb ending -mmin. This gives us huolellisimmin.

Why do we use the elative case ending -stä in hypystä?

The verb laskeutua (to land, to descend) often requires the elative case (-sta or -stä) to express what you are descending or landing from. Here, the boy is landing from a jump, so hyppy becomes hypystä.

Why does hyppy change to hypystä with only one p?

Adding the -stä ending closes the final syllable of the word. This triggers consonant gradation, so the strong double consonant pp weakens to a single p.

I know the verb laskea (to lower or to count). Is laskeutuu related to it?

Yes, it is! While laskea usually means you are lowering something else, adding the -utua ending makes the verb reflexive. So laskeutua literally means to lower oneself, which translates naturally in this context as to land or to descend.