A preposition is always followed by an object. Together they form a phrase, not a clause.
- The match was cancelled because of the rain.
Here the prepositional phrase because of the rain cannot be a sentence alone because it does not have a subject or verb. Like all other phrases, a prepositional phrase must be attached to an independent or dependent clause.
In the example given above, the prepositional phrase because of the rain is attached to the independent clause the match was cancelled. Note that an independent clause can be a sentence on its own.
Prepositions are very important words. There are only a limited number of prepositions in English, so learning their correct usage is not all that difficult.
Prepositions and transitional adverbs
Prepositions are different from transitional adverbs. For one thing, prepositions cannot be removed from a sentence without creating a grammatical error. Transitional adverbs, on the other hand, are not grammatically connected to the sentence and hence can be removed.
- The meeting was postponed due to some unfortunate events.
In the sentence given above the preposition due to connects its object some unfortunate events to the rest of the sentence. If the preposition due to is removed from the sentence it will create ungrammaticality because there will be connection between the phrase some unfortunate events and the rest of the sentence.