formula question

i know there is a way to do this, but i don't remember how. i have a column, that i previously ran a formula through that selected a few cells in the column. for the cells that didn't get selected, i want them to equal the value in column D in the corresponding row. isn't there a formula that i can input in or a find/replace that i could input in w/out having to sort through the column and fill down the formula =D whatever row? i thought i had done it before, but it has been so long, i don't remember. if you could respond with formulas and not code, i would appreciate it

TIA!!