Learn Finnish: Lumilautailetko mieluummin kuin hiihdät?

When comparing two verbs, both verbs must be conjugated in the same person. Here, both lumilautailla and hiihtää are in the second-person singular (lumilautailet and hiihdät). You are structurally asking if you snowboard preferably than you ski.

Why is the question particle -ko attached to lumilautailetko instead of mieluummin?

In Finnish, the question particle -ko or -kö attaches to the specific part of the sentence you are questioning, which is then placed at the beginning. Here, the core question is about the action of snowboarding, so lumilautailetko takes the lead.

Why does hiihtää become hiihdät here?

The verb hiihtää undergoes consonant gradation. When you add the second-person ending -t, it closes the syllable, causing the strong ht to weaken to hd. This is a common pattern to watch out for in type 1 verbs.

Can I use hiihtää to mean downhill skiing?

No, hiihtää specifically means cross-country skiing in Finnish. If you wanted to compare snowboarding to downhill skiing, you would need to use the verb lasketella, meaning the sentence would end with kuin laskettelet.