entscheidend
decisive
Determining the outcome of something; decisive for the result.
Sein Tor in der letzten Minute war entscheidend für den Sieg.
His goal in the last minute was decisive for the victory.

decisive / crucial
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decisive
Determining the outcome of something; decisive for the result.
Sein Tor in der letzten Minute war entscheidend für den Sieg.
His goal in the last minute was decisive for the victory.

'Entscheidend' is the present participle of the verb 'entscheiden' (to decide). So it describes something that is actively 'deciding' or determining a situation. This helps you understand the meaning 'ausschlaggebend' (decisive).
Der Schiedsrichter trifft die entscheidende Entscheidung.
The referee makes the decisive decision.
Very often, 'entscheidend' (decisive/crucial) is used with the preposition 'für' + accusative. This construction indicates what the importance or decisive influence relates to.
Die Unterstützung der Eltern war entscheidend für seinen Erfolg.
The support of his parents was decisive for his success.
'Entscheidend' (decisive/crucial) is often combined with nouns that describe a turning point or an important component. Typical examples are: 'der entscheidende Faktor' (the decisive factor), 'der entscheidende Moment' (the decisive moment), 'ein entscheidender Vorteil' (a decisive advantage), or 'die entscheidende Frage' (the crucial question).
Der entscheidende Faktor für den Sieg war die gute Teamarbeit.
The decisive factor for the victory was good teamwork.
A very common and formal expression is 'von entscheidender Bedeutung sein' (to be of crucial importance). It is used to emphasize the extraordinary importance of something.
Die pünktliche Lieferung der Teile ist von entscheidender Bedeutung für die Produktion.
The punctual delivery of the parts is of crucial importance for production.
important
'wichtig' and 'entscheidend' can both say that something has great importance. 'wichtig' is more general: it can refer to many things that one should pay attention to. 'entscheidend' is stronger and more precise. It means that something determines the outcome or result. Something can be important without deciding everything on its own. 'entscheidend' fits especially well for exams, games, negotiations, or problems when one point makes the difference.

Im Bewerbungsgespräch ist der erste Eindruck oft entscheidend.
In a job interview, the first impression is often decisive.
Eine gute Liste ist vor dem Einkauf wichtig.
A good list is important before going shopping.