solid
Stable, firm, and made with good, durable quality. Often refers to physical things or workmanship.
Das Haus ist auf einem soliden Fundament gebaut.
The house is built on a solid foundation.

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Stable, firm, and made with good, durable quality. Often refers to physical things or workmanship.
Das Haus ist auf einem soliden Fundament gebaut.
The house is built on a solid foundation.

The noun that 'solid' describes reveals the meaning. With physical things like 'Fundament' (foundation) or 'Möbel' (furniture), it means 'stable'. With abstract things like 'Wissen' (knowledge) or 'Finanzen' (finances), it means 'well-founded' or 'sound'.
Der alte Schrank ist noch sehr solide gebaut.
The old cabinet is still very solidly built.
In its physical meaning, 'solid' often describes things that are well made and durable. Typical combinations are 'ein solides Fundament' (a solid foundation), 'solides Handwerk' (solid craftsmanship), or 'eine solide Konstruktion' (a solid construction).
Für den Bau des Hauses wurden nur solide Materialien verwendet.
Only solid materials were used to build the house.
In its abstract meaning, 'solid' describes something reliable and well-founded. Common combinations are 'solide Kenntnisse' (solid knowledge), 'eine solide Ausbildung' (a solid education), or 'solide Finanzen' (sound finances).
Eine solide Argumentation ist für die Debatte sehr wichtig.
A solid argument is very important for the debate.
The meaning 'in Ordnung' (okay) or 'ganz gut' (pretty good) is colloquial. It describes a performance that is acceptable but not brilliant.
Der Film war solide, aber ich würde ihn nicht noch einmal ansehen.
The film was solid, but I wouldn't watch it again.
firm / fixed
'fest' and 'solid' can both describe something as stable and reliable. 'fest' mainly says that something does not wobble, does not break easily, or holds well. 'solid' often additionally emphasizes good, durable quality or a reliable basis. For example, you say 'ein fester Tisch' (a firm table) when it stands steadily, and 'eine solide Arbeit' (solid work) when it has been done properly and reliably. Learners may confuse the words because both sound positive. For simple physical stability, 'fest' is often more natural; for quality, performance, or reasoning, 'solid' fits better.

Für den Preis macht das Fahrrad einen soliden Eindruck.
For the price, the bicycle makes a solid impression.
Das Regal steht fest an der Wand.
The shelf is firmly attached to the wall.