Present Perfect Continuous and Past Simple for finished actions. How to differentiate?

Question

please look at my question carefully. It's not the difference btw Present Perfect and Past Simple or Present Perfect and Present Perfect Continuous. This topic isn't usually covered, but looking at the expanation,we could see such an explanation

Present Perfect - unfinished (I've lived there for four years - still live there) Past Simple - finished (I lived there for four years - most likely I don't live there anymore)

Present Perfect - I've done - likely I've finished the activity Present Perfect Continuous - I've been doing - besides other use, we mean that the activity is still in process - unfinished

I see a contradition in the explanation of the use of Present Perfect and Past Simple - both can be used for finished action. What's the difference then? Only if I use particular tense markers?

I got puzzled after seeing this expanation in the The book Roadmap B1

Use the present perfect simple to focus on the completed action. It often explains how much or how many (including none). The birth rate has fallen since the 1960. (how much) COULD we say the same using the Past Simple if omit since the 19603 - will it mean the same&

Use the present perfect Continuous to focus on an action that lasts for a period of time and/or that is unfinished now. It often explains how long.

Scientists have also been exploring other ways to use the sun‘s energy. (action unfinished)

hope, that's clear

Answer 1

The simple past refers to a past event that happened at a finished past time. The past time must be either explicitly mentioned or understood from the context.

The present perfect has several functions, some of which refer to finished things, and some of which refer to unfinished things. The common thread with all the functions of present perfect is that the speaker is always indirectly talking about the present time. Here's some examples of present perfect sentences with the intended present meaning after:

I haven't had breakfast. (means, "I'm hungry.")
I have lived here for eight years. (means, "I live here.")
I've forgotten my wallet. (means, "I don't have any money with me.")
I've finished two of my projects. (means, "Two of my projects are complete.")
I've seen "Titanic" eight times. (means, "I have the experience of seeing Titanic eight times.")

Answer 2

The present perfect tense is used to talk about the past and the present. But the simple past tense is used to talk about the past.

We can use the simple past for a complete action. We can also use the present perfect for a complete action.

I did. I've done. She bought a camera. She has bought a camera.

We do not use the present perfect with yesterday, five minutes ago, last year, a few days ago etc. We use the simple past with them.

She bought a camera last month. (NOT She has bought a camera last month.)

We use the present perfect with today, this week, this month etc.

He has bought a camera this week.

We can use the present perfect for a state in the present.

I've lived there for four years. (I still live there)

He has worked there since 1960. (He still works there)

We use the past simple for a state in the past.

I lived there for four years. (I don't live there now)

We use the present perfect continuous tense for an action over a period of time up to now.

I've been doing. Scientists have also been exploring...