Why use present perfect?

How do we express that an action happened in the past?

John worked.

This is the past simple. The action happened in the past.

John has worked.

This is the present perfect. The action happened in the past, and it has a consequence in the present.

(Another explanation: the action has a link to the present.)

Every perfect tense expresses a result, an impact, due to an action. When you want to express the result of an action without saying it, use the perfect tense.

An action in the past that is important now.

An action from the past with a consequence in the present.

The focus of the sentence is not the action, it’s the result of the action.

You are sitting with a friend, you want to talk about a film you saw the last weekend.

I’ve seen a great movie! = There is a result of this action = I want to talk about it

I saw a great movie! = It’s not important = no consequence = action finished in the past.

Speakers often use “now” to make it clear that the action has a consequence in the present!

I’ve tried this seven times now!

Have you seen it? = I want to talk about it

Did you see it? = It’s not important, but we can talk about it if you want.

At the police station: I have lost my bag = I need help!!!

Later at the bar with your friend: I lost my bag = it’s not important now, nothing we can do about it.

Have you bought the tickets? = we need the tickets to fly!

Have you finished the email? = There is a consequence. I need the email. I want to read the email.

I reserved the best seats.

This expresses that the action happened in the past. That’s all.

I have reserved the best seats.

This expresses a result, a consequence (which is up to interpretation):
e.g.

It could be one of the above, or all of them! We have to guess.

*images are from The King of Fighters XIII and The King of Fighters XIV, copyright SNK