Not a teacher.
The present perfect is used to describe an event that began in the past and
continues in the present. Except when it doesn't: "I have put the bin out".
The simple past is used to describe an event that began and ended in the past.
"I put the bin out" Both of these refer to the same event.
"I have put the bin out".
"How long have worked for the restaurant?" implies that you were hired in the past, and that you are still working there.
Yes
"How long did you work for the restaurant?" implies that you no longer work at the restaurant.
Yes
If an event started in the recent past (perhaps seconds ago) and is still having an effect, then you use the present perfect.
"When are you going to start your homework?"
This isn't the present perfect. It's the future. What event has just happened?
"I've (I have) already started." (I started the homework in the past, and I am still doing it.)
Yes, but ... "I've already finished it." (Present perfect for an event that happened in the past).
If the recent event ended in the past, then you use the past simple.
Not necessarily "I've already finished it."
"When are you going to start your homework?"
Again, that's the future tense.
"I finished my homework already." (I started and finished my homework in the past."
"I've finished my homework already" is BrE for this.
What this means is that present perfect talks about events that began at an
unspecified time.
"I've been working here since 8am Friday 21st June, 2001"
(Present perfect with specified time)
"King Gustavus Adolphus
fought (past simple) against the power of the mighty Holy Roman Empire from 1630 to 1632." is an event that has a specified time.
Yes
"Ikea
has made (present perfect) stylish, inexpensive, and long-lasting furniture for people all over the world." There is no time specified as to when Ikea started making furniture.
There could be if you added one... all over the world since 1932"
"Sweden
has had (present perfect) a reputation for being very cold, but it
was especially
cold (past simple) last winter." Sweden's reputation for cold continues to this day and began at an unspecified time, so we use the present perfect tense. Last winter is a specified time, so the verb is in past simple tense.
I thought I'd throw some Swedish stuff in there, just for you.
As stated earlier, recency of an event is not important. It is important if the event
continues into the present. Otherwise you would use the simple past tense.
Not necessarily. I've given a few examples where this is not the case. This is another.
Hopefully I have been helpful (present perfect!) and answered (past simple!) your question adequately.