If anyone has taught you to say 'When I was at London', they are wrong. With names of places, the correct preposition is 'in.' In London In Mexico In Africa 'In' has a similar meaning to 'inside'. So you say 'The milk is in the fridge' or 'The money is in the bag', and you also say 'in' with the names of towns, cities, countries and continents. This is because these places have defined borders. When you are in Italy you are within the borders of the Italian state. When you are in Rome, you are within the boundaries of that city. We use 'at' in set phrases such as 'at home', 'at school', and 'at work'. We can also use 'at' for other locations, particularly if you are more interested in the function of the place than the fact that it is an enclosed space. For example, you could say 'Let's meet at the cafe' or 'He's at the bank at the moment. He's getting some money'. In a few very rare occasions, it is possible to use 'at' with the name of a city, but what we actually mean is something different. If you're on a long train journey, at some point in the journey you might, for example, phone home and say 'We're at Oxford now' . But you don't mean the city - you mean the station. Likewise a student might say 'I'm at Cambridge'. But they don't mean the city - they mean that they are a student at the university, indicating an institution rather than a physical location. If you mean the city, you would say 'in'. For example, you might say 'Last year I spent a month staying with a family in Cambridge.' I hope that helps.

Ciao Denise... Parlo italiano un po ', ma non molto. Penso che il problema riscontrato è che, in italiano, si dice "in Italia", ma "a Roma". In inglese, si dice "in Italy" e "in Rome". E così, anche noi diciamo "in London", non "at London". Si dice "in" per i paesi e le città. Si dice "in" o "a" per luoghi come stazioni ferroviarie, aeroporti, ecc Se si dice "in" o "a" dipende dal contesto. For more details, look at this webpage http://www.helping-you-learn-english.com/the-difference-between-at-and-in.html

"in" is probably better to use when referring to a large place or things you actually just go into. In London. In Canada. In the shower. In bed. 'at' I feel like is used more with specific places and events. At my house. At the gym. At Wal-Mart. At the movies. At the concert.

We say ''at'' for a point (in time or space), or a destination : I stopped at the traffic lights, I'll meet you at 9pm; I'll meet you at the airport. We use ''in'' when we are talking about an enclosed space (or time): my sister is in the wardrobe; my girlfriend lives in Spain; I'll phone you in the next hour So we always say ''when I was in London'', never at. Maybe someone else can explain this better.

I guess it is more correct to use "in London" when you are talking about the entire city. You use "at" for very specific locations such as the exact address of a certain location in London (ex. "at 49, Featherstone Street, London")