Restrictive vs. non-restrictive clauses

Understanding the Core Difference

Restrictive (Defining) Clauses:

• Essential to identify WHICH ONE
• Cannot be removed without changing meaning
• NO commas
• Answers: "Which specific one?"

Non-restrictive (Non-defining) Clauses:

• Adds extra information
• Can be removed; sentence still makes sense
• WITH commas
• Answers: "What additional info?"

Meaning changes completely:

• Restrictive: Os alunos que chegaram atrasados perderam a prova
  (Only the late students missed the test)
• Non-restrictive: Os alunos**, que chegaram atrasados,** perderam a prova
  (All students arrived late and missed the test)