You use the singular after the number 1 (and compounds ending in 1, like 21, 31, 51, 101, etc) - долар. After numbers 2-4 (and compounds like 23, 34, 52, 332, etc) you use the nominative plural - долари. The larger numbers (5-20, 100, 1000, and their compounds 120, 1050, 566, etc.) require the genitive plural - доларів.

доларів or долари. Why is the former correct and the latter wrong?