Here is the biggest surprise for every learner. The Finnish you read — books, news, signs, this course — is kirjakieli, written Finnish. The Finnish you hear in cafés, on the tram and among friends is puhekieli, spoken Finnish. It can look like a different language.

Written (kirjakieli)	Spoken (puhekieli)	English
minä / sinä	mä / sä	I / you
minulla / sinulla	mulla / sulla	I have / you have
minun / sinun	mun / sun	my / your
minä olen / sinä olet	mä oon / sä oot	I am / you are
hän / he (people)	se / ne	he, she / they
tämä / tuo	tää / toi	this / that
en tiedä	en mä tiiä / emmä tiiä	I don't know
kyllä	joo	yes

Both are correct Finnish — just different registers, like I do not know and dunno. You should understand puhekieli; you don't have to speak it yet. In this course, spoken forms are always marked as spoken.