Finnish has three registers: kirjakieli (written), puhekieli (spoken) and yleispuhekieli (common spoken Finnish). What each one is, and which to learn first.
There isn't just one puhekieli. Everyday words change between towns and groups of people, which is one reason kirjakieli exists as a standard everyone understands. The short forms change much less: "minä" might be "mä", "mää" or "mie", but it gets shortened almost everywhere.
Some grammar changes too. "Hän on" becomes "se on" and "he ovat" becomes "ne on", even for people. "Me menemme" becomes "me mennään". Possessive endings drop, so "minun kirjani" is "mun kirja". "Vaikea" becomes "vaikee", "tehdä" becomes "tehä", and "haluan" becomes "haluun".
Kirjakieli is what you read: news, books, signs, job applications, and letters from Kela, Migri or the tax office. Puhekieli is what you hear: at work, in shops, on TV series and podcasts, and in most text messages.
SaunaSpeak teaches the short forms first and always shows the written form too.
Literally "book language". It is standard written Finnish: the Finnish of newspapers, books, official letters and most textbooks. Puhekieli literally means "speech language", the Finnish people use when they talk.
No. Puhekieli is the normal way everyone talks, including teachers, doctors and managers. Slang is a separate layer of informal words on top of it, like "kämppä" for a flat or "himaan" for home. You can speak puhekieli all day without using any slang.
In informal writing, yes. Text messages, group chats and social media comments are mostly puhekieli. Anything official stays in kirjakieli: forms, job applications, and letters to a company or an office.
No. In everyday situations it is the normal choice, and pure kirjakieli can sound stiff, like reading aloud. Finns do not expect a learner to get this perfect. If you are unsure, the form closer to kirjakieli is never wrong.
Learn the common spoken forms first and keep kirjakieli next to them. The spoken form is what you will hear. The written form is what you will read, and it tells you what the word really is. SaunaSpeak shows both on every sentence.