Robert-n1 had a very good explanation! I agree with him but would like to add the following: "Upon" is the correct word to use to look back "upon" something, such as childhood. In the vernacular (conversational English), you can say "looking back on my childhood" because "on" is the short version of "upon". EX: Looking back upon my childhood... (more academic/standard English) Looking back on my childhood... (vernacular/conversational English) Hope this further helps in explaining! :)

On is generally used for a surface the picture is on the wall he is working on the floor At is generally used to talk about a specific location he is sitting at his desk

In addition to what Robert wrote; Arthur is "at" work right now. He arrived "on" time this morning. I met his secretary "on" Monday. She said the manager is "on" his coffee break and won't be back by 2. We also use on to mark the time of an event. That's why it is correct to use "on" in comparison to "at"-- looking back on/ upon my childhood. side note: If you're traveling to another country, you don't say "I'm at China". It should be "I'm in China". I know many people who are confused when to use at/ in. E.x. I'm currently having lunch with my friends at some restaurant in Beijing.

You use the following words: At for a POINT, In for an ENCLOSED SPACE, and On for a SURFACE. Please look at the following examples to use word AT: 1. At the corner 2. At the bus stop 3. At the top of the page 4. At the door 5. At the end of the road 6. At the entrance 7. At the crossroads 8. At the front desk 9. At home 10. At work 11. At school 12. At university 13. At the top 14. At the bottom 15. At reception 16. At the side Please look at the following examples to use word ON: 1. On the wall 2. On the ceiling 3. On the door 4. On the cover 5. On the floor 6. On the carpet 7. On the menu 8. On a page 9. On a bus 10. On a train 11. On a plane 12. On a ship 13. On a bicycle or motorbike 14. On a horse or elephant 15. On the radio 16. On a television 17. On the left or on the right 18. On the way Please look at the following examples to use word IN: 1. In the garden 2. In London 3. In France 4. In a box 5. In my pocket 6. In my wallet 7. In a building 8. in a car 9. In a taxi 10. In a helicopter 11. In a boat 12. In a lift 13. In an elevator 14, In the newspaper 15. In the sky 16. In a row 17. In Oxford Street Look at these examples: •Jane is waiting for you at the bus stop. •The shop is at the end of the street. •My plane stopped at Dubai and Hanoi and arrived in Bangkok two hours late. •When will you arrive at the office? •Do you work in an office? •I have a meeting in New York. •Do you live in Japan? •Jupiter is in the Solar System. •The author's name is on the cover of the book. •There are no prices on this menu. •You are standing on my foot. •There was a "no smoking" sign on the wall. •I live on the 7th floor at 21 Oxford Street in London.

Both are OK