Present perfect Study this example situation:
Tom is looking for his key. He can't find it.
He has lost his key.
He has lost his key = He lost it recently, and he still doesn't have it.
Have/has lost is the present perfect simple:
I/we/they/you have (= I've etc.) finished, lost, done, been etc.
He/she/it has (= he's etc.) finished, lost, done, been etc.
The present perfect simple is have/has + past participle. The past parcitiple often ends in -ed (finished/decided etc.), but many important verbs are irregular (lost/done/written etc.).

When we say that 'something has happened', this is usually new information:
* Ow! I've cut my finger.
* The road is closed. There's been (there has) an accident.
* (from the news) Police have arrested two men in connection with the robbery.

When we use the present perfect, there is a connection with now. The action in the past has a result now:
* "Where's your key?" "I don't know. I've lots it." (= I don't have it now)
* He told me his name, but I've forgotten it. (= I can't remember it now)
* "Is Sally here?" "No, she's gone out." (= she is out now)
* I can't find my bag. Have you seen it? (= Do you know where it is now?)
You can use the present perfect with just, already, and yet. Just = a short time ago:
* "Are you hungry?" "No, I've just had lunch."
* Hello. Have you just arrived?

We use already to say that something happened sooner than expected:
* "Don't forget to send the letter." "I've already sent it."
* "What time is Mark leaving?" "He's already gone."

Yet = until now. Yet shows that the speaker is expecting something to happen. Use yet only in questions and negative sentences:
* Has it stopped raining yet?
I've written the letter, but I haven't sent it yet.

Note the difference between gone (to) and been (to):
* Jim is on holiday. He has gone to Italy. (= he is there now or on his way there)
* Jane is back home now. She has been to Italy. (= she has now come back)