Here is a brief review of the form and function of the present perfect tense.
Form
The present perfect is formed like this: have plus past participle
I have studied for three hours today.
Function
The present perfect is used when the time period has not finished.
I have seen three movies this week.
(This week has not finished yet.)
The present perfect is often used when the time is not mentioned.
Gerry has failed his exam again.
The present perfect is often used when the time is recent.
Ikuko has just arrived in Victoria.
The present perfect is often used with for and since.
Greg has lived here for 20 years.
Greg has lived here since 1978.
When you are sure that you understand the lesson, you can continue with the exercises.