what is the function and how and when should I use the if-else statement.. Any help please

Answer 1

when handling a condition or e.g. user input that has 2 possibilities
if(input>0){
/* some operations e.g.
return 3/input;
*/
}
else {/*print error or something like that
can't devide by zero
return null;
*/
}

Answer 2

whenever there is a condition where there are two kind of output or say two possibilities .. for eg
while writing a function for voting...you can vote only if you are above 18 else you cannot vote as you donot meet the minimum age for voting.
so here you can have either of two outputs ..
1) you are allowed
2) you are not

so in this case you can use if else statement .

if (age>18)
{
// print allowed
Console.Write("you are allowed");

}
else {
//print not allowed
Console.Write("you are not alloed");

}