so the -oin represents was In this case yes. Usually it is anyway something -in in imperfect. Example: I ran -> Juoksin. Verbix has good tables for all cases. Others past forms like perfect and plusquamperfect has different and they are done with auxilary verb olla.       and -ssä/ssa at the end same thing but how do you get the -ma before -ssa in laulamassa   Good question, I don't know. After little thinking ending is actually -massa added to end of imperative.