über
above / over
Indicates a position above something, without direct contact.
Die Lampe hängt über dem Tisch.
The lamp is hanging above the table.

over / about
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above / over
Indicates a position above something, without direct contact.
Die Lampe hängt über dem Tisch.
The lamp is hanging above the table.

'Über' is a two-way preposition. With a location (question: where?), the dative is used. With a direction (question: where to?), the accusative is used.
Das Bild hängt über dem Sofa.
The picture is hanging above the sofa.
In most non-spatial, that is figurative, meanings, 'über' is used with the accusative. This applies when it indicates a topic, an amount, or a period of time.
Wir reden über den neuen Film.
We're talking about the new film.
To indicate the topic of a conversation, text, or thought, 'über' is often combined with verbs of speaking, thinking, or feeling. Examples are: sprechen über (to talk about), lachen über (to laugh about/at), sich ärgern über (to be annoyed about), nachdenken über (to think about).
Die Kinder lachen über den Witz.
The children are laughing at the joke.
When 'über' means 'mehr als' (more than), it stands directly before a number or an amount. It is frequently used in news and reports.
Er wartete über eine Stunde auf den Bus.
He waited over an hour for the bus.
To describe a period of time during which something happens, 'über' is used with the accusative. In everyday usage, 'über das' is often contracted to 'übers'.
Übers Wochenende habe ich meine Großeltern besucht.
Over the weekend, I visited my grandparents.
When 'über' has the meaning 'via', it describes the path or medium through which information or an action is transmitted.
Man kann die Rechnung bequem über das Smartphone bezahlen.
You can conveniently pay the bill via smartphone.
from / of
'von' and 'über' can both name a topic. For example, you can say 'von einer Reise erzählen' (to tell someone about a trip) or 'über eine Reise sprechen' (to talk about a trip). 'über' is often the normal and clear choice when a conversation, text, or book deals with a topic. 'von' fits especially with verbs like 'erzählen' when you are referring more to the content or a story. Learners confuse the words because both sometimes have to do with a topic. If you are generally naming the topic of a conversation, 'über' is usually safer.

Im Meeting sprechen wir über das neue Projekt.
In the meeting, we talk about the new project.
Beim Abendessen erzählt Tom von seiner neuen Wohnung.
At dinner, Tom talks about his new apartment.