Which is the second law of thermodynamics? There are many statements. https://en.wikipedia.org/wiki/Second_law_of_thermodynamics#Various_statements_of_the_law
But how do they say the same thing ? I don't understand. Why all of them are the same ?
 A: The statement of the second law of thermodynamics that you have read about are all equally equivalent.  
The one with entropy in it is more used because entropy is a state function and a numerical value can be assigned to its change.  
The other statements of the second law are statements which are more qualitative and are about those processes which cannot happen.  As such they are possibly less used unless the second law is to be introduced without any mention of entropy?
A: The Second Law of thermodynamics simply states that entropy increases.  For example when salt dissolves in liquid water, the amount of entropy or randomness increases with time.  See
A link from Georgia State University
A: The second law of thermodynamics explains the feasibility and spontaneity of a process. It was formulated to overcome the shortcomings of the first law i.e. feasibility and direction.
In essence, all specify similar ideas,


*

*A process in which entropy increases tends to be spontaneous.

*Heat naturally flows from hot to cold in the absence of any work.

*Efficiency is never unity.

