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Infix: The expression in which the operator appears in between the operands is known as infix expression. For example, (a+b), where a and b are operands, + is an operator that appears between the operands a and b. 

Postfix: Similarly, the expression in which the operator appears after the operands is known as postfix expression. For example, ab+, where a and b are operands, + is an operator that appears after the operands a and b.

Postfix: AB + C * DE – FG + * -

Infix: (A+B) * C – (D-E) * (F+G) 

Conversion from postfix to infix:

There is rules/algorithm for converting an expression from infix to postfix. The rules are:

1. The infix expression should be scanned from left to right.

2. If the symbol is an operand then it will be pushed into the stack.

3. If the symbol is an operator then

      a. Pop the top two operand op1, op2 from the stack.

     b. Put the operator between two recently pop operand and form a string 

         like (op1 + op2).

      c. Push the resultant string into a stack.

4. After completion of the iteration when the stack contains only one string then we can treat this string as an infix expression.

5. Return the result.

Input:  AB + C * DE – FG + * - (Postfix)     

So, output: (A+B) * C - (D-E)*(F+G)  (infix)