跟 out-，over-一样，under-也是英语本族语的前缀，它的前身就是能充当介词、形容词与副词的under。派生前缀under-的基本意思主要有三种：under or beneath (在…之处）；lower (低于 …）；not enough (不足）。under-用于前两种意思时，只跟名词和动词缀合；用于第三种意思时只与动词与形容词结合。应注意， 与名词结合时，可能改变词基单词的词性。

下面针对的不同含义分别举例说明。

underground [ under- = under or beneath 在之下;ground n. 地面→]I ad. beneath the surface of the ground 在地下 / Miners work underground. 矿工在地底下工作。II a. being beneath the surface of the ground 地下的/Underground railways play an important role in the modern cities.地下铁道在现代城市里起着重要作用。

underwear [ under- = under 在…之处；wear n. 衣服→] n. clothes worn under the outer garments, especially next to the skin [总称]内衣，衬衣，贴身衣 / She bought some underwear at the winter sales. 她在冬季大拍卖中买了一些廉价的内衣。

underline [ under- = under 在…下面; line v. 用线标示→] v. draw a line under 在…下面划线 / In writing, we underline titles of books. 在写作时，我们在有关书名下划一条线。

understand [ under- =在…下面 ; stand v. 站立=>“stand under what is expected to learn站在希望学到的目标之下”→]v. grasp the meaning of; know well 理解；熟悉 / Now I understand the teacher’s question. 现在我理解老师提的问题了。

understandable a. 可理解的；可懂的