This is the 13817th most frequent German word.
ernsten
"Ernsten" is the dative or accusative plural form of "ernst," meaning "serious" or "earnest."
Here, 'ernsten' functions as an adjective describing the noun 'Ausdruck'. It indicates the characteristic of being serious.
Er hatte einen ernsten Ausdruck auf dem Gesicht.
He had a serious expression on his face.
'Ernstes' is an attributive adjective modifying the noun 'Gespräch', signifying the nature of the conversation.
Wir haben ein ernstes Gespräch geführt.
We had a serious conversation.
'Ernsten' acts as a descriptive adjective in dative case, modifying the noun 'Ton' and highlighting the tone of voice.