Present Perfect

We use the present perfect simple to talk about a time period that is not finished (e.g. today, this week):

I’ve written a rough plan this morning. (it is still morning)

to show that something happened at some point in the past before now. We don’t state when it happened:
I’ve collected plenty of information. (at some point before now and I will use it to write my essay).

The following time expressions are often used: ever, never, before, up to now, still, so far.
It’s the longest I’ve ever had to write, (at any point before now)

If we state when something happened we must use the simple past:
I wasted a lot of time last week, (not I have wasted a lot of time last week)

to talk about a present situation that started in the past, usually with for/since:

I’ve worked really hard for the last two weeks. (I’ve worked hard till now)

We use for with a length of time (e.g. for two hours, for three days, for six months) and since with a point in time (e.g. since 2001, since Monday, since ten o’clock, since I was four, since I started the course).
to talk about something that happened at an unstated time in the past but is connected to the present:
I’ve read all the books on the reading list. (I have the notes now)

The following time expressions are often used: recently, just, already, and yet with negatives or questions.
I’ve just got up.
Have you written your assignment yet?