Defining Relative Clauses

They invented a vaccine. The vaccine helps with the common cold. That vaccine would be implemented next year.

We use who for a person and which for a thing. We can use that to replace who or which.

• This is the presentation which/that we are going to study today.
• Your own examples...

• You are listening to the teacher (= the teacher, in this case, is the object)
• What is the teacher (who/that) saying?

where describes a place: There is a new mall in town where you can buy new clothes.

Whose describe possession The man whose house was on fire is my cousin

• When describes times (year / day / time)
• These days when I can sleep and rest seem to never come

They are non necessary for the sentence to make sense. They just give extra information about the noun. They always have a comma before, and another comma or a full stop after.

• My new cat, who is a tabby, is just 3 months old.

• We cannot use that in a non-defining relative clause
• We never omit a relative pronoun in a non-defining relative clause.