portaisiin
Portaisiin is a Finnish grammatical form used to express movement into multiple stairs or stairwells. It represents the illative plural, a case that marks direction toward a destination, in this instance toward several stairs. The form illustrates how Finnish uses suffixal changes to indicate number and grammatical case, with illative plural endings often appearing as -iin in nouns that take the -iin pattern in the plural.
In terms of usage, portaisiin tends to appear in descriptive or architectural writing, navigation descriptions, or
Grammatical notes include that Finnish illative forms vary by noun stem and sometimes by phonological adjustments
See also: Finnish grammar, illative case, portaat, portaali.