'Have' is only an auxiliary verb when it's used with a verb in a perfect tense (present perfect, past perfect etc.). Ex. Do you have a... Have you done... I had a... I had done... Actually, there ARE sometimes exceptions to this rule, but they sound old-fashioned. So "I hadn't a car" isn't technically incorrect, but it's extremely old-fashioned. "I didn't have a car" is the standard way to say it.

"I did not have a car" is correct.

I hadn't got is perfectly fine. People were giving opinions on italki but I hadn't got clue what they were on about. You know what I mean guv?

We don't use ''hadn't'' in our daily life So i didn't have is the right one

"I didn't have" is the grammatically correct usage. Example: I didn't have any ka-chigs with me so I couldn't buy a car. The other, "I don't had", is incorrect. But you could say, "I haven't had..." Example: I haven't had any fun in a long time. You could also say, "I hadn't had..." Example: I hadn't had apple pie since I was a kid".