Although sometimes tiny two-letter words, prepositions are not as insignificant as they might first appear. They connect the different elements of a sentence. Generally, they are combined with other words to form a structure called a prepositional phrase. Such a phrase begins with a preposition which introduces a noun or pronoun (its object). In the sentence, "Flowers are blooming in the garden," the preposition "in" leads to its object "the garden." Prepositions most often answer these questions: where?, when? and how?
Indicating Movement or Location
"To" is the most basic preposition expressing movement and direction, according to the Online Writing Lab at Purdue University. "Toward," "into" and "onto" can further answer where something is headed. Once movement is complete, more passive prepositions of location identification, like "at," "upon," "under," "over," "between" and "within," take over. Interestingly, "to" can also be used as an infinitive in conjunction with a verb. Sometimes the preposition shows up in both roles in the same sentence: "We flew to New York to visit our father." In this case, "to" explains why, as well as where.
Establishing a Temporal Relationship
Many prepositions of time are part of English usage: "since," "until," "after," "before" and "for," to name several. They denote when an event has happened or will happen or measure its duration. Although each can have other purposes, "at," "on" and "in" sometimes designate times, dates, or in the case of "in," nonspecific times occurring during a day, month, season or year, reports the Capital Community College Foundation. An example would be: "She plans to quit in the fall."
Functioning as Adverbs
When a prepositional phrase indicates under what conditions something happened, it takes on an adverb's role and answers the question of how. Writing "Jack approached the dog without fear," the phrase "without fear" functions as an adverb describing Jack's approach, according to the University of Ottawa's Writing Centre. Generally, prepositional phrases act as adjectives modifying the subject of the sentence or adverbs modifying the verb. In order to give specifics of the action, the how, it must be adverbial.
Conquering prepositions is one of the hardest parts of studying another language. Think about trying to learn English if you were not a native speaker. Someone lies "in" bed but "on" the couch, the Capital Community College Foundation points out. Likewise, an actor appears "in" a movie but "on" television. "At" and "on" are so versatile they can answer either where or when, depending on the phrase, such as "at noon" and "on time" versus "at home" and "on the boat."