Statements under if executes only, if the given condition is true.
Otherwise those statements will not be part of program.
Likewise, when we give some expression or logical combination of expression in if statement, the statements under if only work, if the expressions output is non-zero.
Dividing two numbers. We should restrict the execution, if the divisor value is 0.
If we are going to place only one statement under if, then no need for parenthesis.