The main difference between the two sentences lies in the tense. "didn't have" -> simple past "hadn't had" -> past perfect/pluperfect The first sentence just describes a state in the past. Somebody could, for instance, say this about a person, that they had to work with in the past and now don't. Back then the relationship was not good and now it does not matter since they dont have to see each other anymore. The second sentence alse describes a state in the past, but the pluperfect implies that there has been a more recent "past" in which the state (of your relationship) was different. This sentence, could be said in the context about a person that they didnt like in the beginning, but started to like at a certain point. So your interpretation is right on the money. :)

A relationship means you had some involvement together even if it’s just a co-worker. You’re talking about that involvement. It was either good or bad. Relations means interactions — you saw someone from time to time and you didn’t get along those times. You’re adding up all those interactions to make a determination whether it was good or bad. So I would say a relationship is one whole thing and relations is a series of small things. Relationship is much more commonly used. Relations is rarely used.

They are giving both the same meaning and you could use either. The only difference is I didn’t have... is a statement so does need any further information whereas I hadn’t had a good... is more conversational and you would usually go on to explain more. e.g. I hadn’t had a good relationship with him but now it’s much better.

Good question. I'm a beginner of English, and never l learnt to say "I hadn't had relations within someone ", maybe it's ok to just say "I didn't have relation with someone ", I don't know. 🙂