past tense vs past perfect

hi everyone, as usual i'm not so clear about certain tenses coz i've never gotten any clearest explaination on when to use it and why and the most is how to use it in a sentence properly. i'm confused with this "he had lost his cool and fought with the bullies" why couldn't it be "he loast his cool and fought with the bullies" and another one, why is it like this " they had called him loser,remarking that he was a disgrace to his late father" instead of this "they called him loser,remarking that he was a disgrace to his late father"

i hope i'd really get a very clear explaination of why and how to use it here

thanks