
ernst
serious
Not cheerful, funny, or joking; serious and thoughtful.
Warum machst du so ein ernstes Gesicht? Ist etwas passiert?
Why are you making such a serious face? Did something happen?
serious
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serious
Not cheerful, funny, or joking; serious and thoughtful.
Warum machst du so ein ernstes Gesicht? Ist etwas passiert?
Why are you making such a serious face? Did something happen?

serious
Important, dangerous, or worrying; not trivial.
Die Lage im Krisengebiet ist sehr ernst.
The situation in the crisis area is very serious.

to be serious
To say something not as a joke, but because you really think or feel that way; to have a sincere intention.
Ich meine es ernst, wenn ich dir meine Hilfe anbiete.
I'm serious when I offer you my help.

to take seriously
To regard someone or something as important, credible, or significant.
Man sollte die Sorgen der Mitarbeiter ernst nehmen.
One should take employees' concerns seriously.
In the meaning 'seriös' (serious), 'ernst' often describes facial expression or the way someone communicates. Typical combinations are 'ein ernstes Gesicht' (a serious face), 'eine ernste Miene' (a serious expression), or 'ein ernstes Gespräch' (a serious conversation).
Er hatte während der gesamten Besprechung eine sehr ernste Miene.
He had a very serious expression throughout the entire meeting.
When 'ernst' describes a situation, it means 'schwerwiegend' (serious/severe) or 'gefährlich' (dangerous). Common combinations are 'eine ernste Lage' (a serious situation), 'eine ernste Krankheit' (a serious illness), or 'ein ernstes Problem' (a serious problem).
Der Arzt sagte, es sei eine ernste Verletzung.
The doctor said it was a serious injury.
The expression 'es ernst meinen' means that an intention is sincere. The pronoun 'es' is a fixed part of the construction here.
Glaub mir, ich meine es ernst mit meinem Vorschlag.
Believe me, I'm serious about my proposal.
The combination 'ernst nehmen' (to take seriously) works like a separable verb. The object, meaning the person or thing, comes between 'nehmen' and 'ernst'.
Du solltest diesen Ratschlag ernst nehmen.
You should take this advice seriously.
heavy / difficult
When talking about illnesses, both schwer and ernst can be appropriate. schwer mainly indicates a high degree of severity, whereas ernst places greater emphasis on the fact that the situation is worrying or may have dangerous consequences. schwer is often appropriate for a medical classification, while ernst is suitable when expressing concern about how the illness may develop.

Die Lungenentzündung des Patienten ist ernst, weil gefährliche Folgen drohen.
The patient's pneumonia is serious because dangerous complications are possible.
Der Patient leidet an einer schweren Lungenentzündung.
The patient is suffering from severe pneumonia.