Learn Finnish: Mopon vauhti on hidas. - The moped's speed is slow.

We use the adessive (-lla/-llä) to show how an action is done, like traveling by moped (mopolla). In this sentence, nobody is traveling. We are just describing the moped's property (its speed). Therefore, we use the genitive case mopon to show possession ("the moped's").

What is the difference between vauhti and nopeus for "speed"?

While both mean "speed," nopeus refers to exact, measurable velocity (like km/h). Vauhti is more about general pace, momentum, or the feeling of speed. In everyday speech, if you talk about going at a slow pace, vauhti is the natural choice.

The adjective here is hidas. How does its stem change if we need to add other case endings?

Words ending in -as or -äs change their ending to -aa- or -ää- when a case ending is added. They also undergo "reverse" consonant gradation, meaning the basic dictionary form has the weak consonant (d in hidas), but the stem gets the strong consonant (t). So the stem is hitaa- (for example, the genitive is hitaan).