Prepositions express relationships of time, place, or direction between elements of a sentence (in, on, under, before, after). Conjunctions join words, phrases, or clauses, and come in coordinating (for, and, nor, but, or, yet, so), subordinating (because, although, when), and correlative (either…or) varieties. These function words are the connective tissue of English syntax.
You already know that nouns name things and verbs describe actions or states. But sentences are not just lists of nouns and verbs — they express relationships between them. A noun can be somewhere, happen at a time, be directed toward a destination, or belong to something. A preposition is the word that encodes those relationships. "The book is on the table" — on encodes spatial location. "She arrived before noon" — before encodes temporal relationship. "He ran toward the exit" — toward encodes direction. Prepositions always attach to a noun or noun phrase (called the object of the preposition) to form a prepositional phrase, which then functions as an adjective or adverb modifying something else in the sentence.
Conjunctions do a different job: they join. Where prepositions link a relationship between a noun phrase and something else, conjunctions join two grammatically parallel elements — two words, two phrases, or two clauses — into a larger structure. Coordinating conjunctions (for, and, nor, but, or, yet, so — memorized as FANBOYS) join elements of equal grammatical rank. "She can swim or run" joins two verbs. "He was tired but happy" joins two adjectives. "I came early, and she left late" joins two independent clauses. The rule for coordinating conjunctions is symmetry: both sides must be the same type of grammatical unit.
Subordinating conjunctions do something more powerful: they turn an entire clause into a dependent unit that can't stand alone. "Because she studied, she passed" — "because she studied" is now a subordinate clause functioning as an adverb (explaining why she passed). Other subordinating conjunctions include although, when, while, if, unless, until, and since. The key feature is that the subordinating conjunction signals a hierarchical relationship — one clause is grammatically dependent on the other.
Correlative conjunctions (either...or, neither...nor, both...and, not only...but also) work in pairs to link parallel elements with additional semantic precision. "Either you call or I will" — the either...or pair frames a binary choice. The parallelism requirement is strict: whatever structure follows the first half must be matched by the same structure after the second half. Mastering these connective words — prepositions and all three types of conjunctions — gives you the toolkit to build sentences that express complex spatial, temporal, logical, and causal relationships precisely.