What is the difference in usage between роки and років as they are both plural forms of year
The rule for choosing the correct form is as follows:
- With numbers ending in «один» («одна», «одне»), we use nominative singular: сто один рік '101 year'.
- With numbers ending in «два» («дві»), «три», «чотири» we use nominative plural form with a twist: два ро́ки '2 years', два́дцять три ро́ки '23 years'. It is almost the same as nominative plural (роки́ 'years'), but it has a stress of genitive singular. By combining the ending of nominative plural роки́ and the stress of genitive singular ро́ку, we get the form ро́ки. (If you don't change the stress and just use nominative plural, it will be understandable too.)
- With numbers ending in other words you use the genitive plural form: п’ять ро́ків '5 years', двана́дцять ро́ків '12 years' (note двана́дцять doesn't end in «два»), сто ро́ків '100 years'.
I know it might look complex, but that's what Duolingo is for: just keep strengthening the skill until you're comfortable with using it.