Study this example situation:

Ten minutes later:

Now Tom has found his key. He has it now.

Has he lost his key? No, he has found it.

Did he lose his key? Yes, he did.

He lost his key (past simple)
but now he has found it. (present perfect)

The present perfect (something has happened) is a present tense. lt always tells us about the situation now. 'Tom has lost his key' = he doesn't have his key now (see Unit 7).

The past simple (something happened) tells us only about the past. If somebody says 'Tom lost his key', this doesn't tell us whether he has the key now or not. lt tells us only that he lost his key at some time in the past.

• They've gone away. They'll be back on Friday. (they are away now)
  They went away, but I think they're back at home now. (not They've gone)
• It has stopped raining now, so we don't need the umbrella. (it isn't raining now)
  It stopped raining for a while, but now it's raining again. (not It has stopped)

• ‘I've repaired the washing machine. It's working OK now.’  ‘Oh, that's good.’
• ‘Sally has had a baby! It's a boy.’  ‘That's great news.’

• I repaired the washing machine. It's working OK now.

• Mozart was a composer. He wrote more than 600 pieces of music.
  (not has been ... has written)
• My mother grew up in Italy. (not has grown)

• Did you know that somebody has invented a new type of washing machine?
• Who invented the telephone? (not has invented)

• A : Ow! I've burnt myself.
  B: How did you do that? (not have you done)
  A: I picked up a hot dish. (not have picked)
• A: Look! Somebody has spilt something on the sofa.
  B: Well, it wasn't me. I didn't do it. (not hasn't been ... haven't done)