PREPOSITIONS

Prepositions vs. Conjunctions

Words that are sometimes conjunctions can act as prepositions.

The subordinating conjunctions BEFORE, AFTER and UNTIL can act as prepositions when they are followed by objects rather than dependent clauses.
Remember that a clause has a subject and a verb. A prepositional phrase does not.

Difference between conjunctions and prepositions

Conjunctions are usually defined as words that join words, clauses or sentences together. Prepositions are defined as expressing relations between parts of a sentence.

However, by expressing relations prepositions also join parts.
On the other hand, most conjunctions do in fact also express a relation. The only one that seemingly doesn't is and, although "being in a group" is also an information... just that we see it as the default. It can't be argued though that "but" does indeed express something quite different than "and" in the following example.

Thomas calls Maria and/but he is in a hurry.

So... with conjunctions also relating and prepositions also joining... what is the difference?
Is there a definition that clearly excludes the other group without naming specific words?