Learn Danish: Kendskabet til terrænet er vigtigt. - The knowledge of the terrain is important.

In Danish, til is typically used for relationships, attitudes, or knowledge directed towards something. If you used the -s ending (terrænets kendskab), it would sound like the terrain itself possesses the knowledge! The preposition af is usually reserved for parts of a whole (like the roof of the house) or materials, rather than directed familiarity.

Why do both kendskab and terræn have the definite -et ending here?

When you use a preposition like til or af to show a possessive or relational link, both nouns keep their definite endings if you are talking about specific things ("the knowledge" and "the terrain"). If you used the -s possessive instead, the first noun would have to lose its article. But because we are using this analytic preposition structure, both Kendskabet and terrænet take the -et suffix.

Could I use viden instead of kendskab for "knowledge" in this sentence?

You could say viden om terrænet, but kendskab is much more natural here. Kendskab implies familiarity or practical acquaintance gained through experience (like knowing your way around a physical area). Viden refers more to theoretical or factual knowledge (like knowing academic facts about the terrain's geology). Notice also that kendskab pairs with the preposition til, while viden usually pairs with om.