Sentences & syntax Relative clauses
The information in a defining relative clause is essential as it defines what we are talking about. We can't leave it out. Commas are not used. As a relative pronoun, you can use, for example, 'who', 'which', 'where' or 'that'.
Portobello Market is one of the places that most of the visitors go to.
You can omit it completely when it is the object of the verb.
The old things which they sell here ... → The old things they sell here ...
The information in a non-defining relative clause is extra information which isn't essential, so we could leave it out. The non defining relative clause is separated by commas. You always use a relative pronoun: like 'who', 'which' and 'where', but never 'that'.
This is Jenny.
This is Jenny, who runs the stall.