Prepositions – Small (but Important) Words!

Prepositions: What you need to know.


According to the Merriam-Webster Learners Dictionary, a preposition is: “a word or group of words that is used with a noun, pronoun, or noun phrase to show direction, location, or time, or to introduce an object”.

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Prepositions express relationships such as space (place, position, direction), time, or figurative location. A preposition always has an object (usually a noun or pronoun). The preposition and its object (and any modifiers) are together called a prepositional phrase.

For example:

  • The preposition “on” in “The flask is on the lab bench” shows location.
  • The preposition “in” in “The centrifuge will finish in 3 hours” shows time.


Prepositions are often called the biggest small words in English because although they are generally short words, they are very important to the meaning of the sentence. A misused preposition can make a big difference between a clearly stated sentence and a confusing jumble of words. When used properly, however, prepositions provide the glue between parts of a sentence that allows you to share your scientific research more precisely and professionally.

Prepositions are used to connect nouns, pronouns, or phrases (called the object of the preposition) to other words within a sentence. They reveal the temporal, spatial, or logical relationship of their object to another word or part of the sentence. For example:

  • The flask is on the lab bench. (space)
  • The waste basket is below the lab bench. (space)
  • The centrifuge is beside the lab bench. (space)
  • The fume hood is across from the lab bench. (space)
  • He broke the flask during the experiment. (time)

In each of these examples, the preposition (bold) is used to show the relationship in space or time of one noun (red) to another noun (blue). The second noun (blue) is called the object of the preposition. Note that a preposition can comprise multiple words (e.g., across from).


Prepositions are usually short words, and are normally placed directly in front of nouns, noun phrases, or pronouns. Because they are somewhat vague, learning about prepositions and using them correctly in English sentences takes practice. Here are some rules for using prepositions:

  • Prepositions are followed by a noun, and never by a verb.
  • A prepositional phrase begins with a preposition and ends with the preposition’s object (either a noun or pronoun).
  • The subject of the sentence cannot be part of a prepositional phrase.
  • Verbs cannot be part of a prepositional phrase.

There are hundreds of prepositions in the English language. Understanding how to use each one may seem a bit daunting. Most of these prepositions fall into one of three categories: those denoting space (place, position, or direction), time, or other relationships. Some prepositions are formed using two or three words – like “across from” or “in front of.”


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