A, An, The Are Prepositions?
"A", "an", and "the" are articles. You can read here about their function: https://en.wikipedia.org/wiki/Article_(grammar). They modify nouns directly. As parts of speech, they fall under the heading of adjectives.
Prepositions are another part of speech entirely. They include words like on, of, with, to, at, in, etc., and they generally combine a noun with other words in a way that conveys information about it. https://www.merriam-webster.com/dictionary/preposition#examples