Learn Finnish: Pudotitko ainoan veitsemme pohjattomaan mereen? - Did you drop our only knife into the bottomless sea?

The base adjective is pohjaton (bottomless). Adjectives ending in the caritive suffix -ton or -tön use a strong-grade stem ending in -ttoma- or -ttömä- when adding case endings. Here, we need the illative case for motion into something, so we take the stem pohjattoma-, lengthen the final vowel, and add -n, resulting in pohjattomaan.

Why is the object veitsemme instead of veitsenemme?

The word for knife is veitsi. As a total object, it would normally take the genitive ending -n, becoming veitsen. However, when you attach a possessive suffix like -mme (our), the genitive -n is always dropped. Therefore, veitsen + -mme becomes simply veitsemme.

Does the adjective ainoan also drop its -n because of the possessive suffix on veitsemme?

No, it does not. While the noun veitsi drops its genitive -n before the possessive suffix -mme, any adjectives modifying that noun still keep their standard case endings. Because the object is grammatically in the genitive/accusative case, ainoa takes the regular -n ending, remaining ainoan.