1) you needn't have bought the book. You had no need to buy the book, it was unnecessary. 2) you didn't have to buy the book. You weren't obligated to buy the book, it wasn't required. In general they are used interchangeably and the form "you didn't have to" is more common. Very minor difference, in most situations there is no difference.

Having renting two DVDs

I rented two DVDs today. One is a Japanese anime "kizumonogatari" and the another is a Hollywood movie "JURASSIC WORLD". Since kizumonogatari was only for one hour, I didn't enjoy it very much. But JURASSIC WORLD was longer than kizumonogatari and there were dinosaurs which were scary and made my heart beat so fast. I think I'll watch it again to improve my listening skills.

Who is held responsible for human stupidity?

Today morning five policeman was shot down in the result of a tragic accident when three different police divisions had wanted to detain one of the local gangs. Cops didn't identified each other during the their duty and all the mortal shots were done by policemen.

Who is held responsible for human stupidity?

This morning five policemen were shot down as a result of a tragic accident where three different police divisions wanted to detain one of the local gangs. Cops didn't identify each other during the incident and all of the mortal shots were done by policemen.