Questions & Answers about Tässä romaanissa on paljon sivuja.
Why does sivuja end in -ja instead of just -a?
Because we are talking about multiple pages, we need the partitive plural. The word paljon (a lot) requires the word following it to be in the partitive. The base word is sivu (page). Its singular partitive is sivua, but to make it plural, we insert the plural marker -j- before the ending, resulting in sivuja.
Why is the verb on used instead of ovat, if the English translation says "There are"?
This is an "existential sentence", which simply states that something exists in a place. These sentences start with a location (Tässä romaanissa) and always use the singular verb on, even if there are multiple things there. Furthermore, using a quantity word like paljon (a lot) as the subject also forces the verb to remain in the singular.
Why are both tässä and romaanissa in the inessive (-ssa / -ssä) case?
In Finnish, demonstrative pronouns like tämä (this) have to match the case of the noun they describe. Since romaani (novel) is in the inessive case to say "in the novel" (romaanissa), the word for "this" must also take the inessive ending, becoming tässä.