How do I make a line between the text in a variable

Also, the suggested code wouldn't even compile in C#. I'm guessing you meant var a = "hihihihihi"? or string a = "hihihihihi"? Also, I'm guessing you meant Console.WriteLine not System.WriteLine?

Assuming the guesses are correct, there are two problems. First, you are using Console.WriteLine improperly.

The overload that takes a string argument and a variable number of additional arguments is for formatting strings. In this case, the first argument provides the format and the remaining arguments provide the parameters to be used with that format.

For example, Console.WriteLine("Number={0}", 123) would write out the text "Number=123" followed by a new line character (WriteLine adds a new line onto whatever it writes out).

The second problem (noted by kmn1235) is that you seem to expect the WriteLine method to (somehow) magically separate your string. It will not do this.

For comparison, the following code would provide the output you hope for:

Console.WriteLine("hi");
Console.WriteLine("hi");
Console.WriteLine("hi");
Console.WriteLine("hi");
Console.WriteLine("hi");

Although, I would probably put that in a loop as follows:

for(int index = 0; index < 5; index++)
Console.WriteLine("hi");

you have to split the "hihihi" up before you can put them on separate lines.
If you are trying to put text together to output onto a text file with carriage returns you can concatenate Environment.NewLine on your string as well, otherwise refer to previous answers if you are trying to output to console.