Learn Croatian: Zašto sudiš ljudima? - Why do you judge people?

In English, we judge someone using a direct object, but in Croatian, the verb suditi requires the dative case. You can think of it as passing a judgment to or upon someone. Therefore, we use the dative plural ljudima instead of the accusative ljude.

I know ljudi means "people", but what is the singular word for a person?

The singular form is čovjek (man/human). This is a highly irregular "suppletive" plural, meaning the word completely changes its root in the plural. You will never say "čovjekovi" or "čovjeci"; the plural of čovjek is always ljudi.

How do we decline ljudi in the other cases?

Even though its root is a total exception, ljudi declines quite predictably in the plural. Nominative and vocative are ljudi, accusative is ljude, and genitive is also ljudi. As you see in this sentence, the dative is ljudima, which is also the exact same form used for the locative and instrumental cases.

Is the verb sudiš related to the word for "court"?

Yes, they share the same root! Suditi (to judge) is directly related to sud (court) and sudac (a judge). Grouping these related words together is a great way to rapidly expand your vocabulary at this level.