from “Learn Different”, 3-7-16, New Yorker
The basics of restrictive and non-restrictive clauses come down to whether the clause is essential information (restrictive) or non-essential information (non-restrictive). The common (though at best incompletely correct and at worst incorrect, depending on whom you ask) is that restrictive clauses use ‘that’ while non-restrictive clauses use ‘who’, ‘whom’, ‘which’, etc.
I include the example above not because it is wrong but rather because it nicely illustrates the inherent ambiguity in this oft-misunderstood grammatical structure. On the one hand, it is by no means grammatically incorrect. On the other hand, to my ear (for what it’s worth), it sounds like an example of when ‘that’ would sound better.