Learn Arabic (Modern Standard): كان الملك شخصا مشهورا.

This is the special rule for كان (was)! When you use كان, the predicate (the part of the sentence that gives information about the subject) changes to the "accusative" case. For most indefinite words like شخص (person) and مشهور (famous), this means adding an extra alif (ا) with a double "a" sound (tanween fatha) at the end.

Why does the adjective مشهورا (famous) get the same special ending as the noun شخصا (a person)?

In Arabic, an adjective must strictly match the noun it describes. Because the rule for كان forced the noun شخصا into the accusative case, the adjective مشهورا has to copy it and take the exact same accusative ending so they stay a matching pair.

Why is the word order كان الملك (was the king) instead of "the king was"?

Arabic strongly prefers to put verbs at the very beginning of a sentence. Because كان (was) is technically a verb, it takes the first position. So, the sentence literally reads "Was the king a famous person."

Does the subject الملك (the king) also change its ending because of كان?

No, the subject of كان stays exactly the same as it would in a normal sentence. It remains in the default "nominative" case. كان only changes the end of the sentence (what the king was), leaving the subject alone.