Breakdown of Tyhjennä minibaari, sillä luulin sen tyhjennetyn.
se
it / he / she (of a person) / that
luulla
to think, to suppose
tyhjentää
to empty
sillä
for / because (coordinating)
Questions & Answers about Tyhjennä minibaari, sillä luulin sen tyhjennetyn.
Why is the pronoun se in the genitive form sen here?
In this sentence, sen tyhjennetyn replaces a standard että (that) clause: että se oli tyhjennetty. When you shrink an että clause down into this participle structure, the subject of the action (se) must change into the genitive case (sen).
How does the word tyhjennetyn work to mean "had been emptied"?
This is the past passive referative structure. It is formed by taking the past passive participle of tyhjentää (which is tyhjennetty) and adding the genitive -n ending. Used with verbs of thinking like luulla, this -tyn form communicates that something "was done" or "had been done" by an unnamed party in the past.
Why doesn't the object minibaari have an -n ending?
Because the main verb tyhjennä (empty) is an imperative command. When a total object is the target of a command, it does not take the usual genitive-like -n ending. Instead, it stays in the basic nominative form.
Could I use koska instead of sillä?
Yes, they both mean "because" and are mostly interchangeable here. sillä translates slightly closer to "for" and sounds a bit more formal or literary than koska. Just remember that while koska can start a sentence, sillä can only be used in the middle to connect two clauses.