高(こう)校(こう) koukou時(じ)代(だい) jidai、彼(かの)女(じょ) kanojoは wa数(すう)学(がく) suugakuが ga得(とく)意(い) tokuiでした deshita。 In high school, she was good at math. mathschoolstudyingpast copula (でした)得意 (being good at)~時代 (era/period suffix)が (subject of stative predicate)fronted time adverbialdouble-subject construction