The bomb went off in a subway last week, it's completely destroyed so you can't use it.

went 是动词 go 的过去式，表示"去、移动、发生"等意思。 off 在这里是副词，表示"离开、脱离、中断"的状态。 went off 是一个短语动词，在这里特指"爆炸"的意思，表示炸弹突然引爆。