A preposition is a word placed before a noun or a pronoun to show in what relation the person or thing denoted by it stands in regard to something else. A preposition used in the wrong place can turn the meaning of the sentence on its head. For instance, 'Please don't shout at me: calm down and speak normally' suggests anger and 'She shouted to me from the upstairs window' suggests that someone is calling someone to be heard. Changing prepositions will change the meaning of the sentence. That's how crucial the role of preposition is. A person with a good command over the English language will use prepositions correctly.