English · B1 · grammar · 5 minutes
Non-defining relative clauses
Add optional information with commas and avoid that in this structure.
Understand the choice
The comma changes meaning. My brother who lives in Cork identifies one brother among several; My brother, who lives in Cork, assumes the listener already knows which brother.
Core rule: Non-defining clauses are separated by commas, add nonessential information, and use who/which/whose rather than that.
Notice it in context
My brother, who lives in Cork, is visiting next week.
Mi hermano, que vive en Cork, viene de visita la semana que viene.
The commas mark who lives in Cork as removable background; is visiting remains the main claim.
The library, which reopened in May, now stays open late.
The known library reopened in May and currently has longer hours.
Which-clause adds history without identifying the library.
Dr Lee, whose research we discussed, will lead the seminar.
The already identified researcher owns the work we discussed and will lead.
Whose expresses possession inside optional information.
What to remember
- Non-defining object pronouns cannot normally be omitted.
- Speech marks these clauses with intonation pauses, reflected as commas in writing.
Common trap: Do not use that after the opening comma in a non-defining relative clause.