"Bonan matenon, kara!"
Translation:Good morning, dear!
"Bona mateno" = a good morning (as the subject of a verb)
"Bonan matenon" = a good morning (as the object of a verb)
It is singular but in the accusative case. (Plural would be "bonaj matenoj" in the nominative or "bonajn matenojn" in the accusative.)
-n is the case marker for accusative; -j is the number marker for plural.
"Bonan matenon" is in the accusative here because it's an expression that's short for something like "(I wish you) a good morning", so it's the object of an implied verb.