
das Thema
topic
The subject or central content of a conversation, text, film, or event.
Das Thema der heutigen Konferenz ist die Digitalisierung der Arbeitswelt.
The topic of today's conference is the digitalization of the working world.
topic / theme
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topic
The subject or central content of a conversation, text, film, or event.
Das Thema der heutigen Konferenz ist die Digitalisierung der Arbeitswelt.
The topic of today's conference is the digitalization of the working world.

to get off topic
An expression meaning that someone starts talking about something other than the original main topic.
Wir sollten nicht vom Thema abkommen, sonst werden wir nie fertig.
We shouldn't get off topic, otherwise we'll never finish.

theme
In music, a central, recurring melody or musical idea that is developed in a piece.
Beethovens fünfte Symphonie beginnt mit einem sehr berühmten Thema.
Beethoven's Fifth Symphony begins with a very famous theme.
Certain prepositions are used with 'Thema' (topic/theme). You talk 'über' a topic (accusative), comment 'zu' a topic (dative), or stay 'beim' topic (dative).
Wir haben lange über dieses Thema diskutiert.
We discussed this topic for a long time.
Common verbs combined with 'Thema' are 'ein Thema ansprechen' (to bring up a topic), 'ein Thema behandeln' (to deal with a topic), and 'das Thema wechseln' (to change the topic).
Er hat in der Besprechung ein wichtiges Thema angesprochen.
He brought up an important topic in the meeting.
The expression 'vom Thema abkommen' (to get off topic) is a fixed idiom. The opposite, meaning to stay with the original subject of conversation, is expressed with 'beim Thema bleiben' (to stay on topic).
Entschuldigung, ich bin kurz vom Thema abgekommen.
Sorry, I briefly got off topic.
When 'Thema' is used in connection with music, composers, a symphony, or a melody, it always refers to the musical idea.
Der Komponist variiert das Hauptthema im gesamten Stück.
The composer varies the main theme throughout the entire piece.
thing / matter
Das Thema is the subject matter being discussed. Die Sache refers more to the entire matter that needs to be discussed, clarified, or dealt with. Use Thema for the content of a conversation; when dealing with a problem is the focus, Sache is more suitable.

Das Thema unseres Gesprächs ist die neue Aufgabenverteilung.
The topic of our conversation is the new division of responsibilities.
Wir müssen die Sache mit der neuen Aufgabenverteilung heute klären.
We need to sort out the matter of the new division of responsibilities today.