It has been suggested by EarningStep on several occasions that I should write a post concerning some common prepositions, because like many other people, he seems to have difficulty in deciding which preposition he should use. This is above and beyond the issue of ending a sentence in a preposition, because you need to choose the right word in the first place.
Choosing between in, on, and at can be quite a challenge, especially for people who are learning English as a second language. Bear in mind that these are general rules of thumb and, like so many other grammar rules in English, there will inevitably be exceptions. One of the best ways to improve your grammar is to read as much as possible. Stick to higher quality works as these are less likely to lead you astray with colloquialisms and inadvertent errors.
When to Use “In”
In the context of time, you could say that Obama was elected to office in 2008 and in the month of November. Generally, in refers to being inside of a larger container of some kind. Obama was not elected during the entirety of 2008; he was elected on a specific day in that year. When referring to some event in the future, you could also say that you will be going to Las Vegas in a month.
Similarly, when it comes to referring to an object in space, you can once again imagine this object inside of a larger container. There are many packages in the courier’s truck. You can find Microsoft’s head office in Redmond, Washington. His car is in the parking lot across the street.
When to Use “On”
Generally, you would use “on” when referring to a specific date or a position above something else. For time, you would say that Christmas Day is on December 25. This is for one day in particular, unlike the month and year-sized “containers” in the “in” example above.
When referring to an object in physical space, you can think about the object resting on top of the other object. They placed the cups on the table. You would also ask what is on television, because the TV shows are resting “on top” of the television set itself.
What to Use “At”
You could say that “at” is the most specific preposition of the three, because you would use “at” to refer to a specific time. My dentist appointment is at 3 o’clock. The doors to the arena will open at ten minutes past the hour.
This specificity carries over to indicate a place in physical space as well. The White House may be in Washington, DC, but it is on Pennsylvania Avenue. More specifically, you can find it at 1600 Pennsylvania Avenue.
Practice Makes Perfect
Just as it may take some practice to decide between me, myself, and I, it can take some time for you to learn when to use in, on, or at. These little two-letter words can be the source of much frustration, which is all the more reason why you need to learn how to use them correctly.