flüssig
liquid
In the physical state of a liquid; not solid or gaseous.
Bei 0 Grad wird Wasser fest, darüber ist es flüssig.
At 0 degrees, water becomes solid; above that, it is liquid.

liquid / fluent
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liquid
In the physical state of a liquid; not solid or gaseous.
Bei 0 Grad wird Wasser fest, darüber ist es flüssig.
At 0 degrees, water becomes solid; above that, it is liquid.

'Flüssig' (liquid/fluent) belongs to the word family of 'fließen' (to flow) and 'der Fluss' (the river). The basic idea of flowing also helps you understand the figurative meanings, such as with money or language.
Das Wasser fließt den Berg hinab.
The water flows down the mountain.
You can recognize the meaning of 'flüssig' (liquid/fluent) from the noun it describes. If it refers to money, it means solvent or having cash available. If it refers to language, it means fluent. If it refers to a movement, it means smooth or fluid.
Seine Bewegungen beim Tanzen sind sehr flüssig.
His movements while dancing are very fluid.
In connection with language, 'flüssig' (fluent) is almost always combined with verbs like 'sprechen' (to speak), 'lesen' (to read), or 'schreiben' (to write).
Er hat ein Jahr in Deutschland gelebt und kann jetzt flüssig sprechen.
He lived in Germany for a year and can now speak fluently.
In the financial meaning, 'flüssig' (liquid/solvent) is usually used predicatively with the verb 'sein' (to be). You say 'jemand ist flüssig' (someone is liquid/has cash available) or 'jemand ist nicht flüssig' (someone is not liquid/does not have cash available).
Ich kann dir das Geld erst nächste Woche geben, im Moment bin ich nicht flüssig.
I can only give you the money next week; at the moment I'm not liquid.
firm / fixed
'fest' and 'flüssig' are opposites when talking about the state of substances. 'fest' says that something has a stable form, for example ice, butter from the fridge, or chocolate. 'flüssig' says that something flows and does not keep a fixed shape, for example water, oil, or melted chocolate. What matters here is only this meaning of 'flüssig', not its meanings related to money or language.

Im Topf wird die Schokolade langsam flüssig.
In the pot, the chocolate slowly becomes liquid.
Nach einer Stunde im Kühlschrank ist die Creme fest.
After an hour in the fridge, the cream is firm.