A relative clause is a subordinate clause that begins with a question word (e.g. who, which, where) or the word that. You can use it to modify a noun or pronoun (i.e. to identify or give more information about it).

 
There is a new book that investigates the controversy over political reforms in Hong Kong.
A university is a place where people pursue advanced knowledge in specific academic disciplines.
The lecture theatre in which the inauguration ceremony will be held is now being cleaned.

 
Realative pronouns.

 
What’s the name of the person who/that first landed on the moon?
This is Dr. Perkins, whom we met at a conference in Canada last year.
All students whose registration numbers begin with 374 should immediately go to the library for a tour.

 
Readuction of relaive clauses:
Half of the training sessions (that are) arranged for the athletes have been cancelled due to bad weather.
The foreigner (who/whom) you saw at the party last night is Giorgio Armani!

 
A minimal pair is two words that differ in only one sound.
Minimal pairs are pairs of words whose pronunciation differs at only one segment, such as sheep and ship or lice and rice. They are often used in listening tests and pronunciation exercises. Theoretically it is the existence of minimal pairs which enables linguists to build up the phoneme inventory for a language or dialect, though the process is not without difficulty

 
1- Don't use commas if the adjective clause is necessary to identify the noun it modifies
2- Use commas if the adjective clause simply gives additional information and is not necessary to identify the noun it modifies.

 
I went to my sister who lives in New York .

In this sentence , I have more than one sister and I went to the one who lives in New York and not to the one who lives in Homs..

I went to my sister , who lives in New York .

In this sentence, I have have only one sister.

In the first sentence, who lives in New York is a defining relative clause
or a restrictive clause

As you know defining relative clauses are not separated from the antecedent by a comma. They give necessary information . They are unlike non-defining clauses , which are separated from the antecedent by a comma, and give extra information.

It is wrong to say or write
I went to my father who lives in london.
This means that you have a father who lives in London and another one who lives in Egypt. . ( oh My God ! )

You have to say and write :

I went to my father , who lives in London.

who lives in london. معلومة إضافية عن والدك وهي وليست محددةأو معرفة لأبيك

A relative clause is a subordinate clause that is inrtoduced by a relative pronoun such as which , who ...etc.

Do not mix between a wh - word a nd a relative pronoun.