wahrhaftig
true
Expresses that something is genuine, authentic, and not merely seemingly so. It describes a deep inner truth or quality.
Er ist ein wahrhaftiger Künstler, der für seine Arbeit lebt.
He is a true artist who lives for his work.

true / truly
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true
Expresses that something is genuine, authentic, and not merely seemingly so. It describes a deep inner truth or quality.
Er ist ein wahrhaftiger Künstler, der für seine Arbeit lebt.
He is a true artist who lives for his work.

'Wahrhaftig' (true/truly) belongs to the word family of 'wahr' (true) and 'Wahrheit' (truth). It intensifies the idea of genuineness and authenticity and means more than just 'wahr' (true).
Seine Geschichte klang nicht nur wahr, sondern wahrhaftig.
His story sounded not only true, but genuine.
The function of the word depends on its position. As an adjective, it stands before a noun ('ein wahrhaftiger Freund' — a true friend). As an adverb for emphasis, it often stands at the beginning of the sentence or before another adjective ('Das ist wahrhaftig schön!' — That is truly beautiful!).
Sie ist eine wahrhaftige Expertin auf ihrem Gebiet.
She is a true expert in her field.
'Wahrhaftig' (true/truly) belongs to elevated or slightly old-fashioned language. It is rarely used in everyday colloquial speech; people would more likely say 'wirklich' (really) or 'echt' (real/really).
Das Schloss war ein wahrhaftig königlicher Anblick.
The castle was a truly royal sight.
When 'wahrhaftig' (truly) stands at the beginning of a sentence as an adverb for reinforcement (position 1), the conjugated verb follows directly after it (inversion).
Wahrhaftig, das hätte ich nicht von ihm erwartet!
Truly, I would not have expected that from him!
really
'wirklich' and 'wahrhaftig' can both express that something is real or actually the case. 'wirklich' is very normal in everyday language and fits almost any situation. 'wahrhaftig' sounds stronger, more solemn, or somewhat elevated. It fits when you mean deep authenticity or want to confirm a statement especially strongly. In normal conversation, 'wirklich' is usually more natural. 'wahrhaftig' is used more when something is meant to sound especially impressive or serious.

Nach vielen Jahren stand sie wahrhaftig wieder vor ihrem alten Haus.
After many years, she truly stood in front of her old house again.
Für die Prüfung muss ich wirklich noch viel üben.
I really still need to practice a lot for the exam.