TOEIC・English Undergraduate almost 7 yearsago

なぜ(3)はgoodで（４）はweelなのですか？ わかりやすく教えて下さるとありがたいです

6 ⑬ John speaks better Japanese than Mary、 を Mary (egも) speak as ( 0の ) Japanese as dohn. _ (4) My sister plays the piano better than my brother、 My brother (de play the piano as (ww Q as my Sistet. い) もし