wirklich
really
As an adverb to emphasize or confirm a statement, in the sense of 'actually', 'really', or 'indeed'.
Das Konzert war wirklich fantastisch.
The concert was really fantastic.

really
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really
As an adverb to emphasize or confirm a statement, in the sense of 'actually', 'really', or 'indeed'.
Das Konzert war wirklich fantastisch.
The concert was really fantastic.

'Wirklich' (really) belongs to the word family of 'wirken' (to have an effect) and 'die Wirklichkeit' (reality). So it always has to do with what is real and actual.
Die Medizin hat schnell gewirkt.
The medicine worked quickly.
As an adjective, 'wirklich' (real/actual) stands before a noun and is declined (e.g., 'der wirkliche Grund' – the real reason). As an adverb, it remains unchanged in the sentence and intensifies a statement (e.g., 'das ist wirklich gut' – that is really good).
Was ist der wirkliche Plan hinter dieser Aktion?
What is the real plan behind this action?
As an adverb for emphasis, 'wirklich' (really) often stands directly before the word it is meant to intensify. But it can also stand at the beginning or end of a sentence to reinforce the whole statement.
Das war ein wirklich schöner Tag.
That was a really lovely day.
As a single-word question, 'Wirklich?' (Really?) expresses disbelief or great surprise. It is a common reaction in conversations to question or confirm a piece of information.
Ich habe meinen Job gekündigt. – Wirklich? Warum das denn?
I quit my job. – Really? Why is that?
but
'aber' and 'wirklich' can both make a statement stronger or show amazement. 'aber' often appears as a particle in exclamations, for example when something is surprisingly strong, beautiful, or annoying. 'wirklich', on the other hand, confirms that something is actually the case, or asks whether something is true. Learners may confuse them because both occur in emotional sentences. Remember: 'Das ist aber teuer!' (That really is expensive!) mainly shows a reaction. 'Das ist wirklich teuer' (That is really expensive) confirms or emphasizes the fact.

Ist die Wohnung wirklich so ruhig, wie der Vermieter sagt?
Is the apartment really as quiet as the landlord says?
Das neue Handy ist aber teuer!
That new phone really is expensive!