Learn Finnish: Tietääkseni hän lumilautailee hyvin usein. - As far as I know, he snowboards very often.

This is a special structure based on the verb tietää (to know). It uses the translative case ending which changes to -kse- before a possessive suffix, combined with the first-person possessive suffix -ni. Literally meaning something like "for my knowing," it is a common fixed phrase for "to my knowledge." You can change the suffix for other people, like tietääkseen (as far as he/she knows).

Where does the long verb lumilautailee come from?

It is built from the compound noun lumilauta (snowboard). By adding the verb-forming suffix -illa, we get the infinitive lumilautailla (to snowboard). Because it is a type 3 verb, the third-person singular conjugation doubles the final vowel of the stem, resulting in hän lumilautailee.

Doesn't hyvin usually mean "well"? Why is it translated as "very" here?

While hyvin is the adverb form of hyvä (good) and often means "well," it is also one of the most common ways to say "very" when placed immediately in front of an adjective or another adverb. Here, hyvin usein simply means "very often," just like erittäin usein or tosi usein.

Could I change the word order to Hän lumilautailee tietääkseni hyvin usein?