[Basicz]

pawn Код:

if ( !strcmp( cmdtext, "/rules", true ) )
{
new Str1[ ] =  "1) No cheating, Hacking or using mods to gain unfair advantages.\n\
                2) No racism, flaming, spamming , flooding or behaving like an arsehole.\n\
                3) Do not jack other players who are in vechile mod shop. ( eg at the tune[1-5])\n\
                4) No spawn killing or camping point where the players spawn.\n\
                7) Respect other players and Admins... Do not annoy peoples....\n\
                8) Death Evading is not allowed.\n\
                9) Do not entice people to type /q or /quit.... Its not funny at all.\n",

Str2[ ] =  "Refusing to obey Will result in a mute, kick or a ban.\n\
For any complaints or suggestion please go to our forum sampserver.0fees.net\n",
StrF[ 2600 ];
    format( StrF, sizeof StrF, "%s%s", Str1, Str2 );
    ShowPlayerDialog( playerid, 123, DIALOG_STYLE_MSGBOX , "{00FF19}EFS Rules", StrF, "Close", "" );
    PlayerPlaySound(playerid, 1133, 0.0, 0.0, 10.0);
    return 1;
}

You did it like this

pawn Код:

"This is \n
A string

Which should be

pawn Код:

"This is \n\
A string"

Notice the backslash.

EDIT: I was late ;(
LethaL's way could be good