erneut
again
Once more, repeatedly. Expresses that something happens one more time.
Nach dem Stromausfall musste der Computer erneut gestartet werden.
After the power outage, the computer had to be started again.

again
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again
Once more, repeatedly. Expresses that something happens one more time.
Nach dem Stromausfall musste der Computer erneut gestartet werden.
After the power outage, the computer had to be started again.

'Erneut' (again) is made up of the prefix 'er-' and the adjective 'neu' (new). The prefix 'er-' often initiates a process; here, then, the process of making something 'neu' (new) or repeating it.
Die Firma hat ihr Logo erneuert.
The company renewed its logo.
As an adverb, 'erneut' (again) is flexible. It can stand at the beginning of the sentence in position 1 (then the verb follows), or in the middle field of the sentence, usually after the conjugated verb.
Erneut ist der Zug zu spät gekommen.
Again, the train arrived late.
'Erneut' (again) is often used in written language, in the news, or in formal contexts. It often sounds somewhat more elevated than other words for 'wieder' (again).
Der Politiker betonte erneut die Wichtigkeit des Projekts.
The politician again emphasized the importance of the project.
always
'immer wieder' and 'erneut' are both connected to repetition. 'erneut' means that something happens once more, often in a specific situation. 'immer wieder' emphasizes that something happens several times and keeps repeating. Learners may confuse the words because both are close to 'once again'. If you mean only one additional time, 'erneut' is more precise. If something returns several times or regularly, 'immer wieder' is a better fit.

Nach dem Gespräch ruft meine Kollegin den Vermieter erneut an.
After the conversation, my colleague calls the landlord again.
In der Küche vergisst Tom immer wieder, das Licht auszumachen.
In the kitchen, Tom keeps forgetting to turn off the light.