您问的句子正确.
before 和 ago 都可以用于表示“过去”的时态里.区别是：before 指“从说话时往前”；而 ago 则表示“距离说话时一段时间之前”.例句：
1）He left the city for Peking five days ago.
2）I've never seen her before.
before 也可以用于其他时态,例如：
Before you leave,you'd better take the umbrella with you.