As an explanative conjunction,sillä may be seen as roughly synonymous with the causal conjunctionkoska.
The words are not always interchangeable, andkoska can appear in positions wheresillä cannot. For example, unlikekoska,sillä cannot begin a sentence-initial subordinate clause:
Koska tänään on maanantai, taidemuseo on suljettu.(grammatical)
×Sillä tänään on sunnuntai, taidemuseo on suljettu.(ungrammatical)
In terms of nuance, it can be argued thatkoska implies a stronger causal link thansillä, with the latter instead used to e.g. justify an inference:
Huomenna sataa,sillä niin sanottiin televisiossa. ―It will rain tomorrow because that was told on television.(Nuance: I assume it'll rain tomorrow, based on the fact that the television weather forecast said so.)
Huomenna sataa,koska niin sanottiin televisiossa. ―It will rain tomorrow because that was told on television.(Nuance: the fact that they said it will rain tomorrow on televisionis the reason it will rain tomorrow.)
“sillä”, inKielitoimiston sanakirja [Dictionary of Contemporary Finnish][1] (in Finnish) (online dictionary, continuously updated), Kotimaisten kielten keskuksen verkkojulkaisuja 35, Helsinki:Kotimaisten kielten tutkimuskeskus (Institute for the Languages of Finland),2004–, retrieved3 July 2023
