If it's a meaning you want, 'sternly' means 'harshly' or 'firmly'.
The teacher looked at the misbehaving student sternly before addressing the class.
I can give you several sentences.He spoke sternly to the misbehaving students.I backed down when he glared sternly at me."Follow your orders," the sergeant said sternly.
Example sentence - His father spoke sternly to him when he behaved badly.
Firm or unyielding, uncompromising.
They are synonyms, not antonyms.
Reprimanded
Lamented or sternly
Meaning hard, harsh, or severe in manner or character:
Sternly disciplined, brave, undaunted
it means with sternness; in a severe manner.
grimly, sternly, austerely, seriously, solemnly, thoughtfully
Look at them very, VERY sternly