Learn Dutch: De naad, die erg oud is, wordt steeds zwakker. - The seam, which is very old, is getting weaker and weaker.

This is a non-restrictive relative clause, meaning it just adds extra information about a seam we have already identified. Without the commas, it would be a restrictive clause, meaning 'the specific seam that is old' (implying there might be other, newer seams to distinguish it from).

Why do we use die instead of dat here?

The relative pronoun points back to De naad. Because naad is a de-word, we must use die. If we were referring back to a het-word, we would use dat instead.

Why doesn't the Dutch sentence repeat the word for 'weaker' to mean 'weaker and weaker'?

In Dutch, you generally don't repeat the comparative adjective. Instead, you use the adverb steeds followed by a single comparative (here, zwakker) to indicate an ongoing, gradual change. This is the standard Dutch way to express 'weaker and weaker' or 'increasingly weak'.

Why does the verb wordt appear right after the second comma?

Dutch main clauses follow the V2 rule, meaning the conjugated verb must be in the second grammatical position. The entire subject phrase is De naad, die erg oud is. Since that whole block counts as the first position, the verb wordt must come immediately after it.