“曾”读作 céng 的时候，表示以前发生过的事情，常说“曾经”。

When "曾" is pronounced céng, it indicates something that happened in the past, often used in "曾经" (once).

读作 zēng 的时候，多用在姓，或者表示隔了两代的亲属，比如“曾祖父”。

When pronounced zēng, "曾" is mostly used as a surname or to indicate relatives two generations apart, such as "曾祖父" (great-grandfather).