I want a candy bar. (Any candy bar will do.)
Which one do you want? (Asking for specification)
The one on the right. (I choose that one.)
Give me some milk. (Any milk is fine.)
I need some new shoes. (But I haven’t decided which ones to buy yet.)
I bought some shoes at Valmart. (I know which shoes, but you don’t.)
These are the shoes that I bought. (Now we both know which
ones.)