-isin

From Proto-Finnic *-icin. Related to Estonian -si and Livonian -ž. Probably originally an instructive plural form of *-inen (-inen), at least in the non-temporal sense.

-isin

1. Forms temporal distributive adverbs.

   ‎aamu (“morning”) + ‎-isin → ‎aamuisin (“in the mornings”)

   ‎maanantai (“Monday”) + ‎-isin → ‎maanantaisin (“on Mondays”)

   ‎kesä (“summer”) + ‎-isin → ‎kesäisin (“in the summers”)

2. Used to form some adverbs.

   ‎väki + ‎-isin → ‎väkisin

-isin adverbs are generally considered to be derived from plural stems, but it is likely that the -i- was always part of the suffix, as with -itse.

• -sin
• päivittäin
• viikoittain
• kuukausittain
• vuosittain

• -siin, Sini, sini