One formula to affect multiple rows

New to Calc and Spreadsheets in general so please forgive my ignorance…

I have 3 columns; one for “Item Price” (Col D), “QUANTITY” (Col E) and “Total” (Col F).
If I’m on Row 1 I can enter =PRODUCT(C1:D1) into Column F, Row 1 and it’ll return the product of ITEM PRICE and QUANTITY.
What I’d like to do is to perform the same operation each time a new row is entered…for example Col F, Row 2 will show the product of Col D, Row 2 and Col E, Row 2, and so on as each new line is entered, without having to enter the PROIDUCT formula on each new line.
From what I understand from the HELP files I might be able to enter a formula once that references an array, but at that point I get lost.

Any help? Thank you!

click on the tiny square on right/lower »corner« of F1 and pull down.

Thank you!