
finster
dark
Very dark; with no light or only very little light.
Als der Strom ausfiel, wurde es im ganzen Haus schlagartig finster.
When the power went out, the whole house suddenly became dark.
dark / sinister
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dark
Very dark; with no light or only very little light.
Als der Strom ausfiel, wurde es im ganzen Haus schlagartig finster.
When the power went out, the whole house suddenly became dark.

dark
Describing a gloomy, unfriendly, or threatening mood or expression.
Der Chef machte ein finsteres Gesicht, als er den schlechten Quartalsbericht sah.
The boss had a dark expression when he saw the poor quarterly report.

sinister
Evil, ominous, or shady with regard to intentions or character traits.
Die Gestalt in der Gasse hatte finstere Absichten und beobachtete die Passanten.
The figure in the alley had sinister intentions and was watching the passersby.

dark
Refers to a historical era perceived as backward, unenlightened, or brutal.
Man spricht oft vom finsteren Mittelalter, obwohl diese Zeit auch viele Innovationen hervorbrachte.
People often speak of the Dark Middle Ages, although that period also produced many innovations.
'Finster' (dark/sinister) is often stronger and more emotional than just 'dunkel' (dark). It describes a darkness or mood that is perceived as unpleasant, threatening, or absolute.
Durch die dicken Wolken wirkte der Tag finster und kalt.
Because of the thick clouds, the day seemed dark and cold.
In its literal meaning, 'finster' often describes places that are eerie or completely without light. Typical combinations are 'ein finsterer Wald' (a dark forest), 'eine finstere Gasse' (a dark alley), or 'ein finsterer Keller' (a dark cellar/basement).
Sie traute sich nicht, in den finsteren Keller hinabzusteigen.
She did not dare to go down into the dark basement.
To describe an unfriendly or angry facial expression, 'finster' is almost always combined with words like 'Miene' (expression), 'Gesicht' (face), or 'Blick' (look/glance).
Er warf mir einen finsteren Blick zu, als ich zu spät kam.
He gave me a dark look when I arrived late.
When it is about evil intentions or a bad character, 'finster' is used with nouns like 'Gestalten' (figures), 'Pläne' (plans), or 'Absichten' (intentions).
Im Krimi verfolgte der Detektiv zwei finstere Gestalten.
In the crime novel, the detective was following two sinister figures.
The phrase 'das finstere Mittelalter' (the Dark Middle Ages) is a fixed expression. It is used to characterize this era as backward and unenlightened.
Im Geschichtsunterricht sprachen wir über das sogenannte finstere Mittelalter.
In history class, we talked about the so-called Dark Middle Ages.
angry / evil / bad
Böse generally describes a morally bad or harmful intention. Finster can also describe such intentions, but additionally emphasizes something secretive, ominous, or shady. Use böse for a clear judgment and finster when the intention seems particularly threatening or obscure.

Der Mann hatte einen finsteren Plan, von dem niemand etwas erfahren sollte.
The man had a sinister plan that no one was supposed to find out about.
Der Mann hatte eine böse Absicht und wollte seinen Nachbarn absichtlich verletzen.
The man had evil intentions and deliberately wanted to hurt his neighbor.