hello
im new at assembly (in english too) and i have a question as you may guess: can i add new lines to exe file? its just for experimental causes not for hack, crack or something like that. i wanna protect my software. so i mean if i open exe with hex editor, and insert few bytes (for example opcode of mov command and operand) is program works fine? (okay im sure its bad, very bad grammar. sorry for that) if it can be done, we're not safe at all. anyway, thanks for your helps.

On Windows you have to learn the PE file format, on *nix the ELF format.

Yes it is possible. Best way is to isolate the particular code into its own section, with plenty if padding space to insert code without realigning everything.