Difference between conjunctions and prepositions
The way young children are introduced to prepositions (including multi-word examples) is by being shown pictures illustrating static spatial (locative) relationships ('the box is beside / under / in front of / next to / on top of / on the left of / near (to) ... the table). This is a semantic treatment. Directional relationships come next, then temporal. Non space/time, including the slightly- and highly-idiomatic, usages of prepositions are introduced later, with a necessary re-emphasis on syntactic properties. Prepositions are neither wholly functional nor wholly lexical words, when considered in all their usages.
In Grammar: A Student's Guide, Hurford cites some 'intercategorial polysemes' which nicely illustrate the differences between 'subordinating conjunctions' ('subordinators' may be a more useful term) and prepositions.
Other sources such as
- Brighthub Education
- The Teacher's Grammar of English with Answers (Cowan)
show analyses of prepositions and their use.
The coordinator class is defined purely in terms of syntactic function:
'Coordinating conjunctions, also called coordinators, are conjunctions that join, or coordinate, two or more items (such as words, main clauses, or sentences) of equal syntactic importance.' (Wikipedia) ('Importance' in this sense means one can't have (3) here: (1) 'bacon and eggs is on the menu' (2) 'I like gammon and Jill likes pineapple' (3) 'bacon and Jill likes pineapple'.
'Position in the syntactic hierarchy' is probably a better term.
But if you look at the uses of prepositions Cowan cites, you'll find (I think) they 'relate' structures of unequal syntactic 'importance'.
Conjunction serves the function of addition, I think. it's like a PLUS sign in math. So, it can be replaced with literally "+". It serves as a tool to add two alike parts of a sentence or two words or two sentences. While preposition affects two or more words rather than adding/putting them together. "Three and four equals seven"="three plus four equals seven"...in this sentence "three and four..." the word AND is a conjunction, right? I think so...