1. After the words- are, the same, none, nothing, any, only, the few, the little, etc.
All that glitters is not gold.

2. After adjective in a superlative degree.
Mr Sinha is the most learned man that I have ever met.

3. In speaking of persons, animals and lifeless things.
This is the same person that helped me.

4. After an interrogative Pronoun.
What is that disturbs you?

5. As a substitute for a single noun already mentioned.
The climate of Shimla is cooler than that of Delhi.

6. In an adverbial sense when that implied- when on which etc.
We shall start work the day that you decide.