rückblickend
in retrospect
Looking back on a past period or event and evaluating it from today's perspective.
Rückblickend war es die beste Entscheidung meines Lebens.
In retrospect, it was the best decision of my life.

retrospective
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in retrospect
Looking back on a past period or event and evaluating it from today's perspective.
Rückblickend war es die beste Entscheidung meines Lebens.
In retrospect, it was the best decision of my life.

'Rückblickend' (looking back/in retrospect) is the present participle of the separable verb 'zurückblicken' (to look back). It literally means 'looking backward' and is used to talk about the past.
Der Sportler blickte auf eine erfolgreiche Karriere zurück.
The athlete looked back on a successful career.
Very often, 'rückblickend' (in retrospect) stands at the beginning of the sentence in position 1. In this case, the conjugated verb follows directly in position 2, before the subject.
Rückblickend hätte ich mich anders entscheiden sollen.
In retrospect, I should have made a different decision.
The word is almost always used to evaluate a past situation from today's perspective. A very common fixed phrase is 'rückblickend betrachtet' (considered in retrospect).
Rückblickend betrachtet war der Umzug die richtige Entscheidung für unsere Familie.
Considered in retrospect, the move was the right decision for our family.
belated / subsequent
'nachträglich' and 'rückblickend' both relate to something in the past. 'nachträglich' means that you still do, change, or add something after an event. So it is about an action later on. 'rückblickend' means that you look back at the past from today's perspective and evaluate it. So it is more about thinking, remembering, or judging. The words can be confused because both come after an event. Remember: with 'nachträglich', you do something; with 'rückblickend', you see or judge something differently.

Rückblickend war die Pause nach der Krankheit genau richtig.
In retrospect, the break after the illness was exactly the right thing.
Zu Hause ergänzt Max nachträglich seine Notizen aus dem Unterricht.
At home, Max adds to his notes from class afterward.