When I was in high school, I thought that prepositions are just single-worded part of speech that is why they are really dangerous specially in academic writing. But then I've crossed into some kind called "complex prepositions" that are not single-worded one. Complex prepositions are a word group that functions as a single preposition that may be two-word or three-word..
Some examples are:
In addition to single-word preposition, there are others called complex preposition.
She is a great composer as well as a great Musical Director.
There are garbage in front of the door.