In and At
In is usually used with large places – countries, districts, large cities etc. At is generally used for small and unimportant places like villages, small towns etc.
- We shall meet him at the club this evening.
- His brother lives in Paris.
Notes: This rule is not very rigidly followed. In is often used with small places. At, however, is seldom used for big places.
On, in, at and by
At shows an exact point of time; on shows a more general point of time and in shows a period of time.
- I have a meeting at 4 pm.
- The train leaves at 2 o’clock.
- I was born on a Monday.
- I was born on April 21st.
- I was born in January.
- We will visit them in the summer.
- It is very hot in the day but quite cold at night.
By shows the latest time at which an action will be finished. So it is usually used with a future tense.
- I will be leaving by 6 o’clock.
- I hope to finish the work by next week.
On and upon
On is generally used to talk about things at rest. Upon is used about things in motion.
- She sat on a chair.
- He jumped upon his horse.
With reference to time, in means at the end of a certain period; within means before the end of a certain period.
- I will finish writing this book in three days. (at the end of three days)
- I will finish writing this book within three days. (before the end of three days)