Learn Finnish: Mistä sinä tykkäät?

The verb tykätä (to like) has a special rule called "verb rection". It requires the thing you like to be in the elative case (which usually means adding -sta or -stä). Because of this rule, the question word mikä (what) must also be in the elative case, becoming mistä. Literally, the sentence is asking "From what do you like?".

How do we get tykkäät from the dictionary form tykätä?

Tykätä is a Type 4 verb (verbs ending in a vowel + -ta or -tä). To find the stem for this type, you drop the -tä and add -ää, giving us the stem tykkää-. Notice the consonant gradation: the single k in the basic form strengthens to a double kk in the stem. Finally, add the "you" ending -t to get tykkäät.

Do I have to include sinä in this sentence?

No, you can simply ask Mistä tykkäät?. In Finnish, the personal ending -t on the verb already makes it clear that you are talking about "you" (singular). Including sinä just puts a little more emphasis on the person, like asking "What do you like?".

If I wanted to use the other verb for "to like" (pitää), how would this sentence look?