Always make sure your preposition has an object. What might appear to be a preposition is actually another type of word if it does not have an object. An object is the noun that shows what the preposition's relationship applies to in space or time. In the sentence "The hat is inside the box," the word "box" is the object. Without "the box," the word "inside" would not be a preposition because there is not an object to describe the box's relationship to another thing.
Avoid using long strings of prepositional phrases. A prepositional phrase is a preposition, an object and sometimes other words describing the object. Putting too many of these phrases together in one sentence can render it confusing and difficult to decipher. If you are prone to writing many prepositional phrases in each sentence, consider paring them down to make your writing or speaking clearer.
Look at a prepositional phrase as a modifier of the main sentence. It does not include the main noun or verb. Its noun is the object of the preposition, not the subject of the sentence. Yet, a prepositional phrase can increase confusion over subject-verb agreement. "The dogs run in the yard," shows correct subject-verb agreement. However, consider the prepositional phrase in "The dogs of the family run in the yard." The verb "run" has to agree with the subject, "dogs," rather that the object of the preposition, "family." If not, you would come up with the incorrect sentence ("The dogs of the family runs in the yard").
Avoid ending a sentence with a preposition unless it does not sound natural to say the sentence any other way. There is an anecdote in which Winston Churchill provided a comeback to those who say you can never end a sentence with a preposition: "Ending a sentence with a preposition is something up with which I will not put." In Strunk and White's "Elements of Style," the authors make a case that the only way to know whether to end a sentence with a preposition is to hear how it sounds. In formal writing, do not end a sentence with a preposition unless your ear tells you otherwise.
Learn the difference between prepositional phrases and coordinating conjunctions. A coordinating conjunction might use the same word as you would see in a prepositional phrase. However, it is not being used as a preposition. Rather, it is being used to connect two groups of subject and verb. An example of the conjunction use is "He spoke after we ate dinner." An example of a prepositional phrase is "He spoke after dinner." Notice that there is no second subject in the last sentence.