She is having an affair with her Japanese teacher on the sly.

她和她的日文老師偷偷有了戀情。

He's been drinking whiskey on the sly for thirty years.

他暗中偷喝威士忌酒已有三十年了。

When she thought no one was looking she opened the cupboard and took a few sweets on the sly.

當她覺得沒人看見時，便打開壁櫥偷偷拿了幾塊糖。