Learn German: Der Passant, der alles sah, war sehr verblüfft. - The passerby who saw everything was very amazed.

verblüfft carries a stronger sense of being dumbfounded, baffled, or flabbergasted compared to the more neutral überrascht (surprised). It implies the passerby was so caught off guard that they didn't quite know how to react.

In this sentence we have der Passant in the nominative case. Does this noun change endings in other cases?

Yes, it belongs to a group of masculine nouns known as weak nouns (or n-declension nouns). While it is der Passant here as the subject, it takes an -en ending in all other cases. For example, if you saw the passerby (accusative), it would be Ich sah den Passanten.

English doesn't use commas around 'who saw everything' because it's a restrictive relative clause. Why does German add them?

Unlike English, German grammar strictly requires commas around all relative clauses, regardless of whether they are restrictive (essential to identifying the person) or non-restrictive (just adding extra background info). The commas separating der alles sah from the main clause are mandatory.

How could we expand this sentence using the current lesson's conjunction, nur dass?