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?

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
'schwer' and 'ernst' can both show that something is not harmless. With illnesses, accidents, or problems, both sometimes fit. 'schwer' often emphasizes how strong or bad the consequence is, for example 'eine schwere Verletzung' (a serious injury). 'ernst' more often emphasizes that you have to take a situation seriously because there is danger or cause for concern. Learners may confuse the words because both occur with problems and illnesses. As a guide: 'schwer' often describes the extent or severity, while 'ernst' describes the importance or danger of a situation.

Die Ärztin sagte, die Lage sei ernst, aber nicht hoffnungslos.
The doctor said the situation was serious, but not hopeless.
Nach dem Sturz hatte der Radfahrer eine schwere Verletzung am Bein.
After the fall, the cyclist had a serious injury to his leg.