Referring to an Entire Clause
Normally, the relative pronoun joka is used to refer back to a specific noun. However, when you want to refer back to an entire preceding sentence or event, you must use the pronoun mikä.
| The magician does a new trick, which is a surprise. | Taikuri tekee uuden tempun, mikä on yllätys. |
| The rabbit disappears, which is impossible. | Jänis katoaa, mikä on mahdotonta. |
| The magic wand appears, which surprises the audience. | Taikasauva ilmestyy, mikä yllättää yleisön. |
Notice how mikä doesn't refer to the "rabbit" or the "wand" themselves, but rather to the surprising event of them disappearing or appearing.
Like all Finnish pronouns, mikä changes case depending on its grammatical role in the relative clause. For instance, if the relative clause describes what followed from the previous event, you will use the elative form mistä (from which).
| The magician found the magic wand, from which a new trick followed. | Taikuri löysi taikasauvan, mistä seurasi uusi temppu. |
Referring to "Kaikki" (Everything)
The pronoun mikä is also required when referring back to indefinite pronouns like kaikki (everything) or words representing superlatives. In these expressions, you will most often see it in its partitive form, mitä, especially when it represents the object of the relative clause.
| Everything that the magician says is magic. | Kaikki, mitä taikuri sanoo, on taikaa. |
| Everything that the trick requires is one white rabbit. | Kaikki, mitä temppu vaatii, on yksi valkoinen jänis. |