Prepositions and postpositions, together called adpositions (or broadly, in traditional grammar, simply prepositions) are a class of words used to express spatial, temporal, {categorical and cognitive}* relations (in, under, towards, before) or mark various semantic roles (of, for).
*My additions
A preposition or postposition typically combines with a noun phrase, this being called its complement, or sometimes object. A preposition comes before its complement; a postposition comes after its complement. English generally has prepositions rather than postpositions – words such as in, under and of precede their objects, such as in England, under the table, of Jane – although there are a few exceptions including "ago" and "notwithstanding", as in "three days ago" and "financial limitations notwithstanding.”

__The Apple Dictionary

Over the years I’ve grappled with clarifying a simple and useful definition of preposition. Recently, I have perceived the wealth of prepositions that express a spatial or temporal relation between two parts of a sentence. Now I see, for the first time, a definition that says this (see above).

I want to add to this an observation that gives a quick and simple overview of prepositions that, so far, has given me an easy handle on this word class.

I think the preposition can be labeled as being a particular type of conjunction. It is the conjunction of (among other categories) space and time. As such, it expresses as a part of speech of foundational importance. (Do Heidegger’s monographs on Dasein boil down to his use of prepositions?)

I now know that the study of any language grammar is, if you will, the layperson’s approach to making a study of logic. If you can say it, you can think it. This leads me to a claim that language is the medium of reason and reasoning, and these, in turn, lead me to the speculation that cognition also has for its medium, language. In this context, I note that language is expanded in scope to include all of the sensory forms of signification (sight, sound, touch, taste, smell).

Language and logic are synonyms. This boils down to saying you can’t practice cognition outside of language.

Grammar is logic, and logic is concerned with the parsing of continuity, so language, which is narrative continuity = logic = spacetime.

Now we’ve arrived at Wittgenstein’s principled cosmic silence.

From here it’s a short walk to making a promising speculation about why exists the problem of evil.

Human life (if not all cognitive life) must intersect with evil because humans cannot keep silent. The cosmic breaking of silence erupts with the advent of the narrative. Herein lies the advent of evil because the presence of narrative unavoidably admits deception, the centerpiece (along with the anti-empathy of selfishness) of evil.

Amidst all of this bother lies the preposition, a signifier of spacetime, spacetime being the ultimate conjunction.