• The relative clause tells us which person or thing the speaker means.ie, they define the antecedent

The man who lives next door is a singer
I worked for a company that went bankrupt

• We do not use commas

• The relative doesn´t tell us which person or thing the speaker means . We already know. They give extra , additional information.

  My brother , who lives in Ireland, is a doctor
We went to Bali, which is is a paradise

• We use commas(,)

• We can use "that"

 The man who/that lives next door is a singer
I worked for a company which/that went bankrupt

• We can leave out the relative pronoun when it's he object.

          We went to an island (that/which)I had seen in a postcard

• "Whom" is not often used. It's too formal

• We cannot use "that"

We went to Bali, which is is a paradise

• We cannot leave out "who" or "which"

• You can use "whom" when it´s the object

Yesterday I met Mike, whom/who I hadn´t seen for ages