I is a variable, so writing is the content of the output variable, and String is output, which means the type of I is string.

console . writeline(" I ")；

"I" is just a string, but you didn't declare variables or refer to variables. I will output it directly here.

console . writeline(I . ToString())；

Without quotation marks, I is a variable. If the variable is not of string type, call its method ToString (). The ToString () method returns its string form.

console . writeline(“{ 0 }”，I)；

What is inside the quotation marks is actually formatted content. Starting from 0, {0} {1}, fill this space with the following variables. You could say that. Just write it down.

Console.writeline ("Arabic numeral zero is output here: {0} Arabic numeral one is output here: {1} Arabic numeral two is output here: {2}", 0, 1, 2);

0, 1, 2 can become a variable. Is that clear?

If not, you may need to combine all variables into a string variable and then output it, which will be more troublesome.