Getting Below the Surface
Many of the difficulties encountered in learning to master the various ways in which below is translated into Spanish have to do with whether its various solutions are due to an adverbial or prepositional use. To complicate matters a bit, one adverbial usage takes into account the position of another object or point of reference and one does not.
In the case of an adverbial use of below that is not dependent on the position of something else, abajo is used, meaning below as in underneath, or even downward; but again, not relative to the position of something else. (The opposite of abajo is arriba). Note that in the following examples, the word abajo, performs the function of an adverb, modifying the verb, indicating in what manner or direction the subjects were looking:
Abajo los astronautas podían ver la Tierra (The astronauts could see Earth below).
El equilibrista se cayó porque miraba abajo (The tightrope walker fell because he looked down).
When there is a reference to something else, that is, when the position of something else matters (stated of simply necessary for the sentence to make sense), then the word debajo is used, meaning below in the sense of under (the opposite of debajo is encima).
Lo voy a meter debajo (I'm going to put it underneath) -- clearly there must be some other object under which something (lo) is being placed.
Juan es muy engañoso; encima se ve muy honesto, pero debajo es mentiroso (Juan is deceptive; on the surface, he looks very honest, but underneath, he's a liar).
Finally, the prepositional usage of debajo simply requires de to be added: debajo de. It is also sometimes rendered as bajo (a true preposition, not to be confused with abajo, an adverb, as seen above):
Debajo de la superficie, hay muchos peces (Below the surface, there are many fish).