【词组解析】
1. at one time 曾经,一度;同时
（1）At one time they used to mine coal in these valleys. 从前他们在这些峡谷中采煤。
（2）The song had a great vogue at one time. 这首歌曾风行一时。

2. at present 目前,现在
（1）No more workers are being taken on at present. 目前不雇请更多的工人。
（2）There are no openings for secretaries at the bank at present. 现在银行里没有秘书空缺。

3. at someone's disposal 任...处理
（1）The rest of the day was entirely at his disposal for reading and recreation. 当天剩余时间完全由他支配,看看书,消遣消遣。
（2）He placed unreservedly all the data he has collected at my disposal.他把他搜集到的全部数据毫无保留地让我使用。
相关解析：disposal
              n. （事情的）处置；（自由）处置权；清理；排列
              adj. 处理（或置放）废品的

4. at the cost of 以...为代价
（1）The lessons learned at the cost of blood helped to sober us. 血的教训使我们的头脑清醒了。
（2）We must cherish experience acquired at the cost of blood. 我们必须珍惜用鲜血换来的经验。

5. at the mercy of 任凭...摆布
e.g.（1）The enemy were at the mercy of us. 敌人在我们面前毫无办法。
（2）The poor stationer was at the mercy of those from whom he borrowed money.这可怜的文具商被他的债主们捏在了手心。
相关解析：mercy
             n. 宽容；怜悯；幸运；侥幸

【练习】
英译中：
（1）At one time we met frequently.
（2）I can't use my office at present; it is under repair.
（3）You have all these things at your disposal.
（4）He rescued the boy from drowning at the cost of his own life.
（5）A future lived at the mercy of terrible threats is no peace at all.