Differences between Past simple & Past perfect (simple)
From my point of view the sentence "I have written 3 pages of my essay today." means, that I haven't finished it yet.
Some work is done ("I have done something already.")
and I will work on it later in the future.
And the sentence "I wrote 3 pages of my essay today." means, that my essay is done, takes 3 pages. The writting ended in the past.
We use the Present perfect when there is a connection with the present.
Really complicated to explain, maybe some examples could help.
Here they are:
1.
Study this axample situation: Nick is looking for his key. He can't find it. He
has lost his key.
("He
has lost his key." means that he lost it a short time ago and he still doesn't have it.)
(This example is from the book "Grammar in Use" by Raymond Murphy)
2.
Have you ever been to United States?
No,
I haven't, but
I have been to the Great Britain.
(This is very common phrase and means that you didn't take a trip to the US, but you can chage it and take the trip in the present or close future.)
(By the way, everyone please correct my mistakes, THX. It is very hard to check my punctuation, because I'm from the Czech Republic, and here are lot of rules of writting commas and they are different from English, so I hope you will help me with that.)