der Erfolg
success
The positive result of an effort, plan, or action. It refers to achieving a set goal.
Das neue Marketingkonzept war ein voller Erfolg für die Firma.
The new marketing concept was a complete success for the company.

success
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success
The positive result of an effort, plan, or action. It refers to achieving a set goal.
Das neue Marketingkonzept war ein voller Erfolg für die Firma.
The new marketing concept was a complete success for the company.

The word 'Erfolg' (success) is closely related to the verb 'erfolgen' (to happen, to occur). The logic behind it is that success 'follows' from an action or is the result that comes from it.
Der Plan war gut, und der Erfolg erfolgte wie erwartet.
The plan was good, and success followed as expected.
'Erfolg' (success) is often combined with the verb 'haben' (to have). You can also use adjectives like 'groß' (great), 'klein' (small), or 'überraschend' (surprising) to describe the success.
Mit ihrer Idee hatte die junge Unternehmerin großen Erfolg.
The young entrepreneur had great success with her idea.
The combination 'mit Erfolg' (with success) means 'successfully' and is used like an adverb to describe that an action led to the desired goal.
Er hat die Prüfung mit Erfolg bestanden.
He passed the exam successfully.
In this meaning, 'Erfolg' (success) is often used with the verbs 'sein' (to be) or 'werden' (to become) to describe a product, work of art, or person as successful.
Das Buch war von Anfang an ein großer Erfolg.
The book was a great success from the very beginning.
result
'der Erfolg' and 'das Ergebnis' both have to do with the outcome of an action or a plan. The most important difference is: an 'Erfolg' is positive and means that you have achieved a goal. An 'Ergebnis' is neutral. It can be good, bad, or simply a piece of information. Learners easily confuse the words because an 'Erfolg' is often also an 'Ergebnis'. But not every 'Ergebnis' is an 'Erfolg'. If you want to judge that something was successful, 'Erfolg' fits. If you are only saying what comes out in the end, 'Ergebnis' is better.

Nach vielen Wochen Training war der erste Platz ein großer Erfolg.
After many weeks of training, first place was a great success.
Das Ergebnis des Tests bekommen wir morgen per E-Mail.
We will get the result of the test by email tomorrow.