text: veintiun or veintiuno? if i say "they cost twenty-one dollars," would it be "cuestan veintiun dolares" or "cuestan veintiuno dolares"? how can i tell when to use each one? "cuestan vientiun dolares" is right one. you can answer ventiuno when they ask you, for example, your age, how many days to this or that and it applies to other things too but i can't put my finger on them right now.