I know that for example if I want to ask someone to call another person at any moment in the future I should use at, an example: "Could you call a taxi at 6 pm" but let's imagine that it is 4 pm and I want the person to call the taxi at the moment so that it shows up at 6 pm should I still use at or should I use for?
1 Answer
If you say "Could you call a taxi at 6pm", it is assumed they will call the taxi at 6pm, not before then. However, if you say "Could you call a taxi for 6pm," that would be the appropriate phrase to get someone to call a taxi for the future - at the current time. So "for" is your best bet.