Present perfect x simple past
Hi everybody! :)
I was taught that if we specify the moment an action takes place, I should use the simple past. I would never say "I have bought a car yesterday" for instance. The correct form of saying it is "I bought a car yesterday" and it seems quite obvious for me. But when it comes to "today", "this year", "this week", I'm no longer certain about it.
I don't know which one I should use in the phrase below.. are both possible? Do they have different meanings? Could you please give me a context for each one?

1) I've already talked to my grandparents today OR 2) I already talked to my grandparents today?

I've found these sentences on the internet (but I'm not sure if the sources are reliable):
3) Can't even wash my hair- *I've already done it today* and I've run out of shampoo. Hope Santa brings me some.
4) As for the script update, *I already did it today*. Maybe you could not see it due to old browser cache. Thanks again!

Are they interchangeable in this contexts?
Thank you!

Question

Present perfect x simple past
Hi everybody! :)
I was taught that if we specify the moment an action takes place, I should use the simple past. I would never say "I have bought a car yesterday" for instance. The correct form of saying it is "I bought a car yesterday" and it seems quite obvious for me. But when it comes to "today", "this year", "this week", I'm no longer certain about it.
I don't know which one I should use in the phrase below.. are both possible? Do they have different meanings? Could you please give me a context for each one?

1) I've already talked to my grandparents today OR 2) I already talked to my grandparents today?

I've found these sentences on the internet (but I'm not sure if the sources are reliable):
3) Can't even wash my hair- *I've already done it today* and I've run out of shampoo. Hope Santa brings me some.
4) As for the script update, *I already did it today*. Maybe you could not see it due to old browser cache. Thanks again!

Are they interchangeable in this contexts?
Thank you!

Answer

Simple past: the event started in the past, and ended in the past.
Example: I talked to my grandparents yesterday for 1 hour from 10:00 a.m. until 11:00 a.m.
The 1 hour 'talk' started in the past and ended in the past.

Present perfect is slightly different.  The event started in the past, and the ending is not clear.  The ending of the event can be anytime up until now.
Yesterday at 10:00 a.m. you started talking to your grandparents...
Today I ask you: "Have you talked to your grandparents?"  I'm not asking if you finished talking to your grandparents, only if you have talked to them at all.  
You would respond with "Yes, I have talked to them."  You are saying that, at some point in the past, you started talking to them.  The length of the talk isn't important, only whether you talked to them or not.

So, to answer your questions:
1) I have already talked to my grandparents today. (present perfect)
This looks good.  At some point today, you started talking to your grandparents. The end of the talk isn't important-only that you talked to them, already.

2) I already talked to my grandparents today. (simple past)
The confusion here is that you use the word 'today'.  Today means the entire day and is not specific.  It would be better to say:  "I already talked to my grandparents this morning."  
This means that your 'talk' started this morning and ended this morning.

Answer

Both ways work in this case (for today, this year, this week etc.), but I'd say that simple form is more common (probably because it is less words).

Answer

I read an article today or an answer from Su.Ki where it was explained that present perfect is something that happened in the past and is still having an effect on the present.

I've already talked to my grandparents today.  That conversation is affecting me still or I am still thinking about the conversation.

I already talked to my grandparents today.  We had a conversation.  Nothing about it is something I am using now in a decision.

Answer

This is a question of aspect: how the speaker sees his action.

If he wants to emphasize the outcome, the result --> present perfect.
- I have already washed my hair today ? (Can't you see?)

If he wants to emphasize that the action was done in the past and that now it is over --> simple past.
- I already washed my hair today. (Now I can do something else.)

This is true also when already is used with recently, once, before, this morning...