die Angelegenheit
matter
A matter, topic, or problem that has to be dealt with or taken care of.
Das ist eine private Angelegenheit, die ich nicht öffentlich besprechen möchte.
That is a private matter that I don't want to discuss publicly.

matter / affair
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matter
A matter, topic, or problem that has to be dealt with or taken care of.
Das ist eine private Angelegenheit, die ich nicht öffentlich besprechen möchte.
That is a private matter that I don't want to discuss publicly.

The word 'Angelegenheit' (matter) is derived from the verb 'angehen' (to concern / to be someone's business). Something that 'angeht' (concerns or has to be taken care of by a person) is an 'Angelegenheit' (matter).
Was die Nachbarn machen, geht uns nichts an; das ist ihre Angelegenheit.
What the neighbors do is none of our business; that's their matter.
'Angelegenheit' (matter) is often combined with adjectives that describe the type or urgency. Common combinations are 'eine private' (a private), 'eine berufliche' (a professional/work-related), 'eine dringende' (an urgent), or 'eine heikle Angelegenheit' (a delicate matter).
Bitte entschuldigen Sie mich, ich habe eine dringende Angelegenheit zu erledigen.
Please excuse me, I have an urgent matter to take care of.
Common verbs used with 'Angelegenheit' (matter) describe dealing with a topic. These include 'eine Angelegenheit klären' (to clarify/settle a matter), 'eine Angelegenheit regeln' (to sort out a matter), or 'sich um eine Angelegenheit kümmern' (to take care of a matter).
Können Sie sich bitte um diese Angelegenheit kümmern?
Could you please take care of this matter?
The word 'Angelegenheit' (matter) tends to belong to formal or written language. It often sounds more official or serious than, for example, the word 'Sache' (thing/matter).
In dieser Angelegenheit wird die Geschäftsführung bald eine Entscheidung treffen.
In this matter, management will make a decision soon.
case / fall
'der Fall' and 'die Angelegenheit' can both mean a matter or problem that people talk about or that needs to be resolved. 'der Fall' fits better when you mean a specific situation, an occurrence, or also a police or court case. 'die Angelegenheit' sounds more like a topic, matter, or task that someone has to deal with or take care of. Learners may confuse the words because both often appear in serious or official situations. In practical terms: a 'Fall' is considered or investigated; an 'Angelegenheit' is discussed, clarified, or taken care of.

Diese Angelegenheit besprechen wir morgen mit Frau Becker.
We will discuss this matter with Ms. Becker tomorrow.
Die Polizei untersucht den Fall seit Montag.
The police have been investigating the case since Monday.