Learn Finnish: Mä kävelin huvipuistoon ja me ostettiin rannekkeet. - I walked to the amusement park and we bought the wristbands.

In colloquial Finnish (puhekieli), the passive form is practically always used to mean "we". Instead of the standard written form me ostimme, everyday spoken Finnish uses me plus the passive past tense: me ostettiin.

How do you build the spoken past tense ostettiin from the basic verb ostaa (to buy)?

For most basic verbs ending in -a or -ä (Verb Type 1), you form the past passive by changing that final vowel to an -e- and adding -ttiin. So ostaa changes its final -a to -e-, and then takes -ttiin to become ostettiin.

Does using the spoken me ostettiin change what case the object rannekkeet is in?

Because rannekkeet (wristbands) is plural, it actually looks the same here as it would in written Finnish! Plural objects always take the -t ending in both active and passive sentences. However, if you bought just one wristband, the passive rule would kick in: a singular object drops its -n ending with a passive verb, so it would be me ostettiin ranneke.

Is mä just a slang word for minä?