A preposition is a word that shows how two words are related. Generally a preposition connects a noun or pronoun with another noun or pronoun. Here are a few examples:

• Ms. Levy sent the card in the mail.
• We will go to the store after the appointment.
• The child hid under the table. 

Notice how the italicized prepositions in the above examples demonstrate the relationship between different parts of the sentence. Prepositions can describe location or direction, express time, or even establish the manner in which something is done. 

• Location or direction: 
  • My favorite store is on Main Street. 
  • Pat jumped through the hoop. 
  • Why are you driving toward the highway?
• Time
  • All of the history students finished the exam before me. 
  • Please turn in the assignment by Friday. 
• Manner
  • Sharon went to the wedding without her friend. 
  • Do you prefer to travel by plane or by bus?

The previous examples are all one-word prepositions, but in English there are many that consist of more than one word. These are just a few examples:

• I went to the store and bought socks as well as shirts. 
• We flew to New York by way of Philadelphia. 
• They’re in this mess because of your actions. 

Lastly, keep in mind that English is much more flexible than other languages when it comes to placement of prepositions in a sentence. To illustrate this, compare these two sentences: 

• Who are you looking for? (informal, spoken English)
• For whom are you looking? (formal, written English)

It is more common to hear the first sentence when walking down the hall at your office and offering your help finding a specific person. The second sentence would be reserved for written English. Many languages (Spanish and German, just to name two) do not allow sentences to have dangling prepositions (prepositions at the end of a sentence). This means that you might have to reposition a preposition within a sentence or at the beginning of a question when writing and speaking in the second language. 

Resources:

• Video introduction to prepositions (3:48)
• Video on prepositions of time (5:15)
• Video on prepositions of space (5:09)