For more details see the TString Reference.

Get substring of existing string

TString new = old(start, subStrLength);
// start is the index of the first character
// subStrLength is the number of characters to copy
 
// e.g: remove S charachters form start and E from end
TString new = old(S, filename.Length()-S-E);

Combine some variables with a static Text

TString string = Form("My Number is %d", number);

If your variable is a TString, you need to pass the underlying char array

TString string = Form("My TString is %s", tstring.Data() );

The function follows printf-syntax