Questions & Answers about Tämä joki on pidempi.
How do we get pidempi from the basic Finnish word for "long"?
The basic adjective is pitkä (long). To form the comparative, we usually take the word's stem and add -mpi. However, pitkä is heavily irregular! The -kä drops off entirely, and an e is added to make the stem. Then consonant gradation changes the strong t to a weak d, resulting in pidempi.
I have also seen the word pitempi. Is that the same thing?
Yes! For the word pitkä, both pidempi and pitempi mean "longer" and both are perfectly correct in standard Finnish. pidempi applies consonant gradation (changing the t to a d), while pitempi leaves the strong t in place. You can use whichever you prefer.
The sentence just says "longer." How would I add "than that river" to this sentence?
The simplest way is to use the word kuin (than) followed by the basic nominative case: Tämä joki on pidempi kuin tuo joki. At this level, you might also start seeing the other method: dropping kuin and putting the compared thing into the partitive case right before the adjective, like tuota jokea pidempi.
Why is pidempi in the basic nominative case here, rather than the partitive case?