Learn Finnish: Jänis ilmestyi ja taikuri katosi, mikä oli outoa. - The rabbit appeared and the magician disappeared, which was strange.

In Finnish, you use joka when referring to a specific preceding noun. However, when the relative pronoun refers to the entire preceding sentence or clause (in this case, the whole event of the rabbit appearing and the magician disappearing), you must use mikä.

Why is the word for strange, outoa, in the partitive case rather than the nominative outo?

When an adjective describes an abstract concept or a whole sentence rather than a concrete noun, it takes the partitive case. Since outoa describes the entire preceding event represented by mikä, it must be in the partitive.

What is the dictionary form of katosi, and why does it have a 't' instead of a 'd'?

The dictionary form is kadota (to disappear), which is a type 4 verb. Type 4 verbs undergo reverse consonant gradation, meaning the weak 'd' in the dictionary form changes to a strong 't' in conjugated forms, such as the past tense katosi.

Is the comma before mikä mandatory?

Yes. In Finnish, a relative clause starting with a relative pronoun like mikä or joka must always be separated from the main sentence by a comma.