"I have switched off the radio." "Why did you switch it off?"
Why can`t present perfect be used in the question? Or if it can what is the difference in meaning?
For some reason only the question with past simple is correct in the test book. Could you please explain the difference in other situations?
Whilst the combination of the two is not incorrect and it is perfectly understandable, I must admit that I would expect to find:
I switched off the radio.
Why did you do that?
I have switched off the radio.
Why have you done that?
Having said that, they're both unnecessarily wordy. You would, in everyday conversation, hear:
I've switched off the radio.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.