Used to introduce a clause or to refer to a specific person or thing.

जो

English Usage: The book that I read was fascinating.

Hindi Usage: जिस किताब को मैंने पढ़ा वह दिलचस्प थी।

Used to specify a thing or person previously mentioned or known.

वह

English Usage: That is the car I bought last year.

Hindi Usage: वह मेरी बीती साल खरीदी गई कार है।

Used to introduce a subordinate clause.

कि

English Usage: She said that she would come.

Hindi Usage: उसने कहा कि वह आएगी।

To that extent; so.

इतना

English Usage: I didn't know he was that good.

Hindi Usage: मुझे नहीं पता था कि वह इतना अच्छा है।