adjective
BAD : The earnest students never missed a class.
GOOD : The serious students never missed a class.
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Earnest is used to describe someone who takes life too seriously, often failing to see things that are humorous: 'He's one of those very earnest types that go around looking for problems to solve.'
Serious is used to describe someone who is fully committed to something: 'I see her at the karate club once a week, but I don't think she's serious.'