The general form of 1809 is:

1 x 1000 + 8 x 100 + 9

The general form of a 3-digit number can be expressed as follows: 

abc = 100a + 10b + c, where a is the digit in the hundreds place, b is the digit in the tens place, and c is the digit in the one's place. Similarly, we can derive the general form of a 4-digit number as follows:

1809 = 1 x 1000 + 8 x 100 + 0 x 10 + 9

= 1 x 1000 + 8 x 100 + 9