Learn Finnish: Yhtäkkiä joku kuulee oudon askeleen. - Suddenly someone hears a strange step.

joku refers to people (someone or somebody), while jokin refers to things or animals (something). Since a person is hearing the sound, we use joku.

Does the pronoun joku always take a third-person singular verb like kuulee?

Yes. Grammatically, joku works just like hän (he/she). That is why we use the he/she form of the verb kuulla (to hear), which is kuulee.

Why do we say oudon askeleen with an -n ending rather than using the partitive case?

The -n ending marks a total object. It shows that the person hears one whole, specific step. If they were listening to continuous stepping sounds, we would use the partitive case. Notice that the adjective outo (strange) must match the noun and also take the -n ending to become oudon.

How do we get the form askeleen from the basic word askel?

askel is a noun ending in a consonant. To add the object ending -n, we first need its vowel stem. For this word type, the stem is formed by adding -ee-, making the stem askelee-. Then you just add the -n.