jīng / guò / jǐ / nián / de / nǔ / lì / ， / tā / zhōng / yú / gǎi / tóu / huàn / miàn / ， / chéng / le / yí / gè / duì / shè / huì / yǒu / yòng / de / rén / 。

经过几年的努力，他终于改头换面，成了一个对社会有用的人。

After years of effort, he finally turned over a new leaf and became a useful member of society.