A preposition shows the relation of a noun or pronoun used as its object to some other word or words in the sentence or, as it has been otherwise stated, makes the noun or pronoun to which it is joined equivalent to an adjective or an adverb. The expression "John is behind the press" is equivalent to an adjective describing John. That is, he is "John behind-the-press." Prepositions are governing words and the words governed by or depending on them are always in the objective case.
A conjunction is the coupling link between the parts of a train of thought. It is of no purpose whatever except to connect.
I am cold and hungry and tired and I am going home.
Care should be taken to avoid confusing and and but and and and or.
He sees the right and does the wrong.
should be
He sees the right but does the wrong.
The ideas are contrasted, not associated.
I did not see Thomas and John.