Could somebody explain to me. What's the difference between: "last week I didn't have smth", and "last week I haven't smth". When I must say "did"? And if I sad "last week I haven't smth", it will be grammar fault? Can opponent understand me?
Yes, you can use both "didn't have" and "hadn't" (not "haven't" because the time is "last week"), but you cannot use "something" in a negative sentence unless you were using "something" as a temporary replacement.