Replace fifth space with new line
Using += to concatenate strings in a loop scales very badly. Each iteration creates a new string instance by copying the previous string and the new character into a new string, and that string grows for each iteration. Using a StringBuilder (as Dmitry suggests) would be a great improvment.

However, you don't need to build the string character by character. You could use a regular expression to replace the fifth space with a newline:

string t = Regex.Replace(s, "^([^ ]+(?: [^ ]+){4}) ", "$1" + Environment.NewLine);


Explanation of the pattern:

^       mathces the beginning of the string
(       starts a catching group
[^ ]+   matches the first word (a sequence of non-space characters)
(?:     start a non-catching group
space   matches a space
[^ ]+   mathces a word
)       ends the non-catching group
{4}     repeats the group four times
)       ends the catching group
space   matches the fifth space


The $1 in the replacement string gets the value caught by the group.



Another alternative would be to loop through the string and look for spaces, then use Substring to get the parts before and after the fifth space:

int cnt = 0, index = 0;
while (cnt &lt; 5) {
  if (s[index++] == ' ') cnt++;
}
string t = s.Substring(0, index - 1) + Environment.NewLine + s.Substring(index);