In your interviews for an ESL teacher, you are asked a question of this sort: 

What is the difference between:

a. I am taking the bus tomorrow.  
b. I am going to take the bus tomorrow. 
            
Or

a. I lived in Kerala for 2 years.  
b. I used to live in Kerala for 2 years.

Surely, they mean the same thing. That's precisely what makes it difficult to explain the difference. Could anyone help me with it?