How to add new line in a string?

Quote:

but i want wrapping if index word not completed then move whole word into next line

There is no function that will do the job for you.
Your code split insert a newline at fixed position when you want to do it at beginning of last word within range of that position.
The code should look like:

if (a.Length > 20) {
    pos= 0; // to remember position of last non alpha char found
    for (scan= 0; scan < 20; scan++) {
        if (char at index scan is not alpha) { // exact code is left to you as an exercise
            pos= scan;
        }
    }
    a = a.Insert(pos+1, Environment.NewLine);
}

Note that solution will depend on the exact usage you want:
- number of newlines (single of multiple)
- long word handling

There's no method that's going to do it for you. You're going to have to write your own method to do it.

How would you do it on paper? How would you look for the spot to place the NewLine? Then you write code to do what you did on paper.

To add to what Dave says, remember that there things you need to check:
1) The input may need "breaking" multiple times to fit in your length. For example "Bill Gates founded the software company Microsoft in 1975" will not fit in two 20 characters strings.
2) A single word may exceed your length: there are seventeen 21 character words in the English language, five 22 character words, and so up, right up to 45: "Pneumonoultramicroscopicsilicovolcanoconiosis". There are also hyphenated words which can easily exceed 20 characters.

But there are two ways to do this:
1) Use string.Split to break the input into individual words, and then assemble strings which are all the right length to fit.
2) Use string indexing ("hello"[1] is 'e' for example) and string.Substring to extract "short" lines by working back from a non-whitespace character to the word start. You can then either use a StringBuilder to assemble your output, or a List to collect them, and then string.Join to build your final output.