
instrumental
Refers to music that is played only with instruments and without singing.
Die B-Seite der Single war eine instrumentale Version des Liedes.
The B-side of the single was an instrumental version of the song.
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Refers to music that is played only with instruments and without singing.
Die B-Seite der Single war eine instrumentale Version des Liedes.
The B-side of the single was an instrumental version of the song.

Playing a decisive or essential role in making something happen; being very important for an outcome.
Ihre Unterstützung war instrumental für den Erfolg des Projekts.
Her support was instrumental in the success of the project.

In grammar, describing a case that expresses the means or tool of an action.
Im Russischen wird das Werkzeug einer Handlung im instrumentalen Fall ausgedrückt.
In Russian, the tool of an action is expressed in the instrumental case.
You can easily recognize the meaning of 'instrumental' from context. If it is about music, the first meaning is meant. If it is about the success of a project, the second is meant. In the context of grammar, the third is meant.
Ich höre beim Arbeiten gerne instrumentale Musik.
I like listening to instrumental music while working.
In the figurative meaning, 'instrumental' is often used in the fixed expression 'instrumental für etwas sein' (to be instrumental in/for something). The noun after 'für' is in the accusative.
Ihre Unterstützung war instrumental für den Erfolg des Teams.
Her support was instrumental in the team's success.
In a musical context, 'instrumental' often appears with words like 'Version' (version), 'Stück' (piece), 'Musik' (music), or 'Begleitung' (accompaniment).
Der Song wurde auch als rein instrumentales Stück veröffentlicht.
The song was also released as a purely instrumental piece.
The grammatical meaning of 'instrumental' is a technical term from language studies (linguistics). This meaning does not occur in normal everyday usage.
In der Vorlesung haben wir die Funktion des Instrumentals in slawischen Sprachen besprochen.
In the lecture, we discussed the function of the instrumental in Slavic languages.
decisive / crucial
Entscheidend emphasizes that something determines the outcome. Instrumental describes an essential role in achieving an outcome and highlights the contribution as a means to success. A role can be instrumental and thereby also become entscheidend.

Ihre Kontakte waren instrumental für das Zustandekommen des Vorschlags.
Her contacts were instrumental in bringing about the proposal.
Ihre Stimme war entscheidend für die Annahme des Vorschlags.
Her vote was decisive for the proposal's acceptance.