First you need to be able to identify what place each number is in.
The 8 is the whole number, so the ones place. The 3 in the tenths. The 5 in the hundredths. The 2 in the thousandths.
Now, you can expand.
(8 + .3 + .05 + .002)
Because we identified what place each number was in, when we expanded the number, we knew where to put the number and to fill in the empty places with zeros.
I hope you understand it the way I tried to explain ! (:
