"I do not know who he is."
Translation:Mi ne scias, kiu li estas.
In a previous question, it asked to translate "you don't know what to say" and accepted the translation "vi ne scias kion diri".
Why is it "kiu li estas" in this sentence instead of "kiun li estas"?
I thought if the kio/kiu are the object of "scias", then they should be in the accusative case. One works, and the other doesn't. Why not?
The object of "scias" are the whole subordinate clauses "kiu li estas" and "kion diri" respectively.
In the clause "kiu li estas" 'kiu' and 'li' are both nominative since they are linked via the copula 'estas'. In "kion diri" 'kion' is in the accusative because it is the object of 'diri'.
I hope that explanation helps!?
Generally speaking, the nominative is the case of the subject, and the accusative is the case of the direct object.
In Esperanto the nominative isn't marked, while the accusative is marked with an -n
Nominative: viro, knabo, patroj, domoj
Accusative: viron, knabon, patrojn, domojn
Copula comes from Latin and means 'tie' or 'belt'. It's called that, because it ties two nouns (or a noun and an adjective) together. In Esperanto the verb 'estas' ist the copula.
Long story short: Never use the accusative with the verb 'estas'.