when can we use simple past instead of present perfect?

Question

here are some examples from a grammar book :

you can use the present perfect for new information:

1. "I've decided to quit my job." "Really? Why?"
2. We've sold our car. Now we take the bus every day.
3. They're having a party next week, but they haven't invited me.
4. I'm looking for Amy. Have you seen her?

you can also use the past in these examples:

1. I decided to quit my job

why just example 1 can be written in past form? and actually, by this fact it should not!

Present Perfect. This tense describes something that happened in the past, but that is related to something in the present.

in general when we can use simple past instead of present perfect or vice versa?

Answer 1

Where was this grammar book published? American English speakers tend to use either present perfect or simple past when British English would only use present perfect: see this article for more information.

I (British English) would not use simple past unless the time it happened is stated in the sentence, or is clear from the context.

1. I quit my job yesterday
   

2. Q: So, what did you do about your problem at work?
   A: I quit my job.

In example 1, the time is clearly stated. In 2, it's implied, because it happened as a consequence of the problem at work... the questioner knows when that happened.

If make a statement out of the blue, with no time and no context, I would use present perfect:

I have quit my job

This canonical post explains very thoroughly when you would use present perfect.