Present Perfect - "It was sent" or "It has been sent"?
"The letter was already sent" or "the letter has alredy been sent"?
"The grass was already cut" or "The grass has already been cut"?

Which one should I use?

Question

Present Perfect - "It was sent" or "It has been sent"?
"The letter was already sent" or "the letter has alredy been sent"? 
"The grass was already cut" or "The grass has already been cut"?

Which one should I use?

Answer

Here, without context, use the present perfect particularly for exams.  (... the letter has been sent.)

In casual speech, Americans and Canadians often prefer the simple past.  (... the letter was sent.)

Here is a brief overview of when to use the present perfect and the simple past.
https://www.ef.com/ca/english-resources/english-grammar/present-perfect-vs-simple-past/

Answer

The first ones are very direct and work with a yes or no.
Yes, the letter was already sent
Yes, the grass was already cut

The second work a bit further and more friendly/fluently.

Who sent the letter? "The letter has already been sent" to the customer
Why are they cutting the grass? "The grass has already been cut" this week