Learn Swedish: Det är pappan som inte kan låta bli att skvallra.

The phrase låta bli att means "to refrain from" or "to stop oneself from". When combined with inte kan (cannot), the whole expression inte kan låta bli att skvallra literally means "cannot refrain from gossiping". This is the standard Swedish way to say that someone "can't help" doing something.

Why is inte placed before kan instead of after it?

This happens because of a very important Swedish word order rule for subordinate clauses (often taught as the BIFF rule). The word som (who/that) starts a subordinate clause here. In these clauses, sentence adverbs like inte must always come before the first conjugated verb (kan). If this were a normal main clause, it would be pappan kan inte.

Why do we need the word att before skvallra?

While it is true that the modal verb kan is followed directly by an infinitive without att (which is why we say kan låta), the att here belongs to the fixed phrase låta bli att. This expression always requires the infinitive marker att before the following action verb, giving us låta bli att skvallra.

Why does the sentence start with Det är pappan som... instead of just saying "Pappan kan inte..."?