This is the 28th most frequent Finnish word.
sitten
"Then," "after that," "since," or "ago" (context-dependent).
Here, 'sitten' is used to indicate a point in time in the past, paired with a duration (an hour).
Tulimme kotiin tunti sitten.
We came home an hour ago.
In this sentence, 'sitten' functions as a temporal marker to refer to an event proceeding another, orderly.
Mitä teemmekään tämän tehtävän jälkeen sitten?
What will we do after this task then?
Here, 'sitten' acts as a logical connector, transitioning from a condition to an action happening as a result.