Grammar : Present Perfect Tense

Study this sentences :

• Dorothy has killed the Wicked Witch of the East
• How many people has Dorothy met on her adventure?

In the sentences above, we use present perfect tense to express things we have done in our lives or our achievement. We do not usually give the exact time.
Here is the pattern:

Positive	Short forms	Negative
I / you have worked	I/you’ve worked	I / you have not worked
She/he it has worked	She/he it’s worked	She/he it has not worked
We/They have worked	We/They’ve worked	We/They have not worked

Questions	Short answer
Have I / you worked ?	Yes, I have	No , I have not
Has she/he worked ?	Yes, she/he/it has	No , she/he/it has not
Have we/they worked ?	Yes,we/they have.	No ,we/they have not

We often use present perfect with "already" and "yet"
examples :

• The Cyclone has already stopped
• Dorothy has not met the Wizard of Oz yet.

We use already to say something that has happened. Already is usually placed before the past participle.
We use yet to say something hasn't happened. Yet usually placed at the end of the sentence.
(Taken From : English On Sky 2, 2007)