Questions & Answers about Der Rand, welcher sehr breit ist, ist leer.
Why use welcher here instead of der?
welcher is a relative pronoun that means the exact same thing as der in this sentence. It is used primarily in written, formal German as a stylistic choice to add variety, especially to avoid saying Der Rand, der.... In everyday speech, you would almost always just hear der.
How do we know to use the -er ending on welcher?
Like all relative pronouns, welcher takes its gender and number from the noun it refers back to (Der Rand, which is masculine singular). Its case comes from its role inside the relative clause. Here, it is the subject of the clause (welcher sehr breit ist), so it takes the masculine nominative ending -er.
In English, we might not use commas here if we used 'that' instead of 'which'. Why does German require commas?
In German, all relative clauses must be separated from the main clause by commas. English distinguishes between 'defining' clauses (often no commas) and 'non-defining' clauses (with commas), but German grammar requires commas around the relative clause every single time, regardless of its type.
Why is there an ist right before the second comma, and another ist right after it?