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. 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. 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. 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. 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.