This is the 2nd most frequent Finnish word.
että
"That" (used as a conjunction).
In this sentence, 'että' is used as a conjunction to introduce a subordinate clause, indicating the content of what is known.
Tiedän, että hän on täällä.
I know that he is here.
Here, 'että' again serves as a conjunction, providing the statement of what is hoped for.
Toivon, että voisimme lähteä aikaisemmin.
I hope that we could leave earlier.
In this sentence, 'että' connects the result or consequence of being tired, thus functioning as a cause-effect linker.