Decimal arithmetic is place-value arithmetic with a point. For adding and subtracting, the decimal points must line up, which is why the stacked sheets here align every number on its point, even when the numbers have different lengths.
For multiplying, students multiply as if the numbers were whole and then place the decimal point by counting decimal places. For dividing a decimal by a whole number, the point in the quotient goes straight above the point in the dividend.
Questions
Why line up the decimal points?
Lining up the points lines up the place values: tenths under tenths, hundredths under hundredths. Adding digits from different places is the most common decimal error.
Do I need to write zeros at the end?
Writing 3.5 as 3.50 does not change its value. It can make subtraction easier because every place has a digit.