This is the 61st most frequent Finnish word.
sen
"Sen" means "its" or "that" in English, depending on context.
Here, 'sen' is the singular object pronoun 'it,' referring to the movie previously mentioned.
Näin sen elokuvan viime viikolla.
I saw that movie last week.
Here, 'sen vuoksi' is a phrase meaning 'because of that,' with 'sen' referring to the mentioned reason.
Sen vuoksi hän myöhästyi koulusta.
That's why he was late for school.
Here, 'sen' is the possessive determiner meaning 'that,' referring to the book.