Picking the right preposition can be tricky. After all, there are so many to choose from! But prepositions can be vital for building a grammatical sentence.\n\nAs such, you\u2019ll need to know how these terms work if you want to avoid errors in your writing. And in this post, we\u2019re taking a closer look at three common prepositions you need to know: \u201cas,\u201d \u201cat,\u201d and \u201cby.\u201d\nWhat Are Prepositions?\nPrepositions are linking words. In particular, they show us how a word is related to other words in a sentence. For example:\nThe man came from Scotland.\n\u201cFrom\u201d here is a preposition that tells us the relationship between \u201cman\u201d and \u201cScotland\u201d (i.e., that \u201cScotland\u201d is where the man originated). Most prepositions work like this, coming before a noun or pronoun.\n\nBut how about the prepositions \u201cas,\u201d \u201cat,\u201d and \u201cby\u201d? What do we use these words for when building a sentence? Let's take a look.\nAs (Function and Character)\nThe word \u201cas\u201d has several uses. In fact, it isn\u2019t even a preposition most of the time! Usually, it is either an adverb (when making comparisons) or a conjunction (when linking clauses).\n\nHowever, it can also be a preposition. In this case, you would use it before a noun or noun phrase to express the function or character of something:\nMonique works as a proofreader.\nAlan was difficult as a child.\nIn the first sentence above, for instance, \u201cas\u201d points to the type of work Monique does (i.e., proofreading). In the second, \u201cas\u201d tells us who Alan was when the speaker considered him difficult (i.e., a child).\nAt (Location, Position and State)\nThe good news is that \u201cat\u201d is always a preposition, which is simpler than \u201cas.\u201d Unfortunately, it also has many uses as a preposition, so it is still tricky! We won\u2019t try to list all its functions here, but key uses include indicating:\n\n \tA location (e.g., They will be at the party later)\n \tWhen something takes place (e.g., We will get there at 8pm)\n \tA point on a scale or in a process (e.g., We\u2019re at the halfway point)\n \tA state or condition (e.g., I\u2019m good at basketball)\n \tThe object of an action or attention (e.g., He keeps staring at the clock)\n\nMost of the time, then, we use \u201cat\u201d to indicate a location, position, or state.\nBy (Responsibility and Means)\nFinally, we have \u201cby,\u201d which is almost always a preposition. This term has many uses, but some of the most common include indicating:\n\n \tWho performed an action (e.g., The goal was scored by Viduka)\n \tThe means by which something was done (e.g., I travelled by bus)\n \tThe size or amount of a difference (e.g., We improved output by 25%)\n \tA deadline or the end of a time period (e.g., We need it finished by Tuesday)\n \tThat something is next to something (e.g., She found it by the side of the road)\n \tThe time during which something happens (e.g., Most owls hunt by night)\n\nAs with \u201cat,\u201d the variety of uses here can seem confusing. But if you practice using the prepositions above, you should get the hang of them before long!