To underlie something is to be the thing beneath it that it rests on or grows out of — the real cause, the basic idea, the foundation. It is almost always used about invisible things: the assumptions that underlie a theory, the anger that underlies a joke, the economic problems that underlie the protests.

To underlie something當某個東西underlie另一個東西, is to be the thing beneath it that it rests on or grows out of意思是成為底下支撐它、或是它從中生長出來的那個東西 — the real cause, the basic idea, the foundation真正的原因、基本的想法、根基. It is almost always used about invisible things它幾乎都是用在看不見的東西上: the assumptions that underlie a theory支撐一個理論的那些假設, the anger that underlies a joke潛藏在一個笑話底下的憤怒, the economic problems that underlie the protests潛藏在抗議底下的經濟問題.

The subject of the verb is the deep thing; the object is the visible thing on the surface. That order catches students out, so it is worth saying slowly: in 'fear underlies his anger', the fear is deep and the anger is what you see.

The subject of the verb is the deep thing這個動詞的主詞是那個深層的東西; the object is the visible thing on the surface受詞則是表面看得見的東西. That order catches students out這個順序常常讓學生搞混, so it is worth saying slowly所以值得慢慢說清楚: in 'fear underlies his anger'在'fear underlies his anger'（恐懼是他憤怒的底層原因）這句裡, the fear is deep and the anger is what you see恐懼是深層的，憤怒才是你看得到的.

This is a formal, written verb. In speech you would more often say 'is behind', 'is really the cause of', or 'is at the bottom of'. It is common in academic writing, journalism and serious analysis.

This is a formal, written verb這是一個正式、書面的動詞. In speech你在口語裡 you would more often say 'is behind', 'is really the cause of', or 'is at the bottom of'比較常會說'is behind'、'is really the cause of'，或'is at the bottom of'. It is common in academic writing, journalism and serious analysis它常見於學術寫作、新聞報導和嚴肅的分析文章.

There is also a plain physical use — 'a layer of clay underlies the topsoil' — used by geologists and builders, and this is the older, literal meaning the abstract one grew out of.

There is also a plain physical use它也有一個單純、字面上的物理用法 — 'a layer of clay underlies the topsoil''a layer of clay underlies the topsoil'（一層黏土在表土底下） — used by geologists and builders地質學家和建築工人會這樣用, and this is the older, literal meaning the abstract one grew out of而這是比較舊、字面上的意思，後來的抽象用法就是從這裡發展出來的.

Its forms are irregular and worth memorising: underlie, underlies, underlying, underlay (past), underlain (past participle). The present participle 'underlying' has taken on a life of its own as an adjective, and is far more common than the verb itself.

Its forms are irregular and worth memorising它的變化形不規則，值得背起來: underlie, underlies, underlying, underlay (past), underlain (past participle)underlie、underlies、underlying、underlay（過去式）、underlain（過去分詞）. The present participle 'underlying' has taken on a life of its own as an adjective現在分詞'underlying'已經自己發展成一個形容詞, and is far more common than the verb itself而且比動詞本身還常用得多.