Read the examples and study the rules.

a. The student who (that) won the math contest last year is from Hà Tĩnh.

b. Hà Nội, which is the capital of Việt Nam, is famous for its landscapes.

RULES 

1. A defining clause gives information about the noun or pronoun it modifies. Defining clauses do not have commas. We use that instead of who, which in informal style.

2. A non-defining clause gives extra information about the noun or pronoun it modifies and has a commas at the start. In non-defining clauses, we don’t use that.