How can Turing machines loop forever given that the input is finite? How Turing machine can loop forever when input length is always finite? 
I think this is a basic doubt but unable to understand how TM go into infinite loop? I think it can make transition forever on empty string. What are the other cases? Can it make without any empty alphabet ?
It might help to think of Turing machines as a programming language. It's easy to see how a program in your favourite language might loop (its input is finite, too!) so you can translate that to a Turing machine.