Wie habe ich das nur gemacht?

Question

So the question is in the title. Because my German is still very basic, and I only know that "nur" means "only", I was wondering if in this phrasing it acts something like a modal particle? I guess it can be translated as "How did I JUST do that?"

Answer 1

You are basically correct - "nur" is used as a particle here.

See also the Duden dictionary entry for Partikel "nur" (in German)

In questions, this may express confusion, astonishment, uneasiness or worries (Meaning 3 "Verwunderung", "Besorgnis" in Duden), depending on context.

In your example it probably expresses astonishment or wonder, as it refers to an accomplishment.

Answer 2

"Wie habe ich das gemacht?" would be a neutral question, where I ask another person to tell me how I did it. For instance, an illusionist could ask this his audience, after performing a trick, to make them think about how he managed to do it.

Adding the "nur" means that the asking person himself does not clearly know, how he did it, and is very surprised by the outcome. Actually, this "nur" expresses the surprise of the asker.

In English, I would translate it using "on earth": "How on earth did I manage to do this?"