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.
He used to live in England.
Yǐqián tā bù xǐhuan kàn diànshì.
She didn’t like to watch TV before.