兄=brother は=is まじめ=diligent* なわりに=(but) considering (his diligence) 成績=(his) grades/records/results が=are 悪い=bad *まじめ in this sentence is talking about his attitude about STUDYING. and also this is his characteristic!! So 兄はまじめなわりに成績が悪い is (my/someone's) brother is diligent but considering his diligence (his) grades are bad. after "わりに" , you put sentence which is kind of surprising considering what you just said before "わりに". like this "わりに" <(he) gets bad grades.> Your sentence 兄はまじめに勉強したけど成績が悪かった is different meaning. 兄=brother は=(you don't need to translate.it's just connecting who did what) まじめに=seriously 勉強した=studied けど=but 成績=grades が悪かった= was bad. (my/someone's) brother studied seriously but (his) grades were bad. Since you used past tense for verbs, also used the word まじめに as an Adverb, this sentence is specifically talking about the one exam he just had. The original sentence 兄はまじめなわりに成績が悪い is current form, and talking about his diligent characteristic and the fact that the results/grades/scores are unfortunate. similar example. あのレストランは高いわりにサービスが悪い。 あの=that レストラン=restaurant は=is 高い=expensive わりに=(but) considering (the price) サービスが=(their) service is 悪い=bad P.S. I love Sheldon!

兄はまじめなわりに成績が悪い For me,it sounds like a joke. ;) So... I assume that this brother(i don't know whose anyway) is a serious guy.He studies hard and attends class,does homework everyday,in spite of that!!! his grade isn't good!! Lol Hope it helps. :)