etain:保留[持],不忘,记住;雇用,聘请(律师等)
e.g.Eliminate the false and retain the true 去伪存真
retain one's presence of mind 镇定自若.
retain an appearance of youth 保有年轻的外貌.
由此看出,retain的主要强调的是"保留原有的"
reserve:储备; 贮藏; 保存保留; 留下(出,给)推迟; 延迟预定[约]; 租定,预定(座位,房间等).
be reserved for 留作,(专)供...之用
with reserve 有保留
without reserve 无保留
由此看出,reserve主要强调"预留以备用".
e.g.I will make sure the hotel reserve a room for us .
Reserve oil to prepare against war.