Your code can look like this:

char modem_receive() {
   
   char str[10];  // Array for storing string of response from modem.
   int n=0;

   TH1 = 0xFF;    // 19200 BAUD @ 11,059,000 Hz
   SCON = 0x50;   // Serial Mode 1, REN = 1

   do
   {
   RI = 0;           // Reset RI Flag
   TR1 = 1;          // Run Timer1
   while (RI == 0);  // Wait receive data complete
   TR1 = 0;          // Stop Timer1
   if( SBUF != '\n') // If character received is not carriage return, stroe character in array str[]
      { str[n] = SBUF;
        n++;
      }
   }
   while( SBUF != '\n');

   return str;

}

by using the prefix "pre" and suffix "/pre" around your text...
surround the commands in "<>"