1st fragment: - I'm half and half. Me dad's a Muggle. Mum's a witch. (Why not "my dad"?) "me" is a slang British variant of "my". - What's he teach? (Can we do a contraction of "does"?) It's not a standard contraction but people in the UK do use it. I don't recommend copying it! 2nd fragment: That ain't no ordinary cut on your forehead, Harry. (Why do we put "no" here? I didn't know you can do double negation in English) Again, the double negative "ain't no" is is non-standard slang. It's common also in US English. It's wrong and we know it's wrong. 3rd fragment: - You don't want to go making friends with the wrong sort. I can help you there. - I think I can tell the wrong sort for myself, thanks. ( Why do we need “for”? Is it possible to say " I can tell the wrong sort myself" ) "For" is a bit strange here. "by" would make sense. I alone am the agent of the "telling of the wrong sort".

Thanks, Elizabeth!!

That ain't no ordinary cut on your forehead, Harry. (Why do we put "no" here? I didn't know you can do double negation in English) In America, when we use this type of incorrect language knowingly, we are exclaiming the point and we are showing disregard for "proper" speech. We might also be trying to fit in with a group of peers. Such as teenagers in rural areas. If a native speaker uses this type of incorrect language unknowingly, it's usually due to lack of education.