
der Gegenstand
object
A physical, material object or thing that you can see and touch.
Bitte räumen Sie alle persönlichen Gegenstände vom Tisch ab.
Please clear all personal objects from the table.
object / subject
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object
A physical, material object or thing that you can see and touch.
Bitte räumen Sie alle persönlichen Gegenstände vom Tisch ab.
Please clear all personal objects from the table.

subject
The topic or content of a discussion, text, investigation, or school subject.
Der Gegenstand der heutigen Vorlesung ist die moderne deutsche Literatur.
The subject of today's lecture is modern German literature.
'Gegenstand' (object / subject) is made up of 'gegen' (against / opposite) and the old word 'Stand' (from 'stehen' (to stand)). It literally describes something that 'stands opposite' you—whether a physical object in space or an abstract topic you are dealing with.
Ein seltsamer Gegenstand lag mitten auf der Straße.
A strange object was lying in the middle of the street.
The context tells you whether a physical object or an abstract topic is meant. If it is about touching, seeing, or moving, it is a thing. If it is about discussing, investigating, or learning, it is a topic.
Er nahm den glänzenden Gegenstand in die Hand.
He took the shiny object in his hand.
In the meaning of 'Thema' (topic), 'Gegenstand' (subject) is often combined with a genitive to specify the content. This construction is typical of formal written and technical language.
Gegenstand der Untersuchung war die Wasserqualität des Sees.
The subject of the investigation was the water quality of the lake.
As 'Thema' (topic), 'Gegenstand' (subject) is found in fixed expressions. Common ones are: 'Gegenstand der Debatte sein' (to be the subject of the debate), 'etwas zum Gegenstand haben' (to have something as its subject), or 'Gegenstand eines Vertrags sein' (to be the subject of a contract).
Die neuen Sicherheitsregeln waren Gegenstand einer langen Diskussion.
The new safety rules were the subject of a long discussion.
topic / theme
Thema and Gegenstand can both refer to what a conversation, text, or investigation deals with. Thema tends to refer to the general subject area. Gegenstand places more emphasis on the specific subject matter or field being examined.

Gegenstand der Untersuchung ist die tägliche Nutzung sozialer Medien durch Jugendliche.
The subject of the study is teenagers’ daily use of social media.
Das Thema der Untersuchung ist die Nutzung sozialer Medien im Alltag.
The topic of the study is the use of social media in everyday life.