Questions & Answers about Ik koop geen schoenen die te stug zijn.
Is the relative clause in this sentence restrictive or non-restrictive?
It is a restrictive relative clause (beperkende relatieve bijzin) because it defines exactly which shoes you refuse to buy: only the stiff ones. You can tell because there is no comma before die. If you added a comma, it would become a non-restrictive clause, bizarrely suggesting you don't buy shoes at all because shoes in general are too stiff!
Why do we use die instead of dat to refer back to the shoes?
We use die because schoenen is plural. For all plural nouns, the relative pronoun is always die, regardless of whether the singular noun is a de-word or a het-word (though in this case, it is de schoen anyway).
Could I use stijf instead of stug for stiff shoes?
You could, but stug is the more natural choice here. stug means rigid, inflexible, or hard to bend, which perfectly describes stiff shoe material like thick leather. stijf is more commonly used for stiff muscles or a rigid body posture.
As a quick refresher, why is this sentence negated with geen instead of niet?