There are several ways to talk about the past in Mandarin Chinese. To refer to an unspecified time in the past you can use yĭqián.

Yĭqián has two characters: 以前. The first one, yĭ, is the same character found in kĕyĭ (可以) and means “according to.” The second character, qián, means “before.” Yĭqián can be placed either before or after the subject.
Examples of Yĭqián
 
    Tā yǐqián zhù zài Yīngguó.
    他以前住在英國｡
    他以前住在英国｡
    He used to live in England.

    Yǐqián tā bù xǐhuan kàn diànshì.
    以前她不喜歡看電視｡
    以前她不喜欢看电视｡
    She didn’t like to watch TV before.