Present continous - For an action that has started but not yet finished.
I am drinking coffee (now).
= I started drinking coffee (a few minutes ago). I am drinking coffee (now). I will finish drinking eventually (in a few minutes).
I am reading a book (this week).
= I started reading (a few days ago). I may or may not be reading now. I will finish eventually (in a few days).
I am studying Spanish (this year).
= I started studying (a few months ago). I may or may not be studying now. I will finish eventually (in a year, in two years, ...).
Present perfect continuous - For an action that has started but not yet finished
I have been drinking coffee since five minutes ago.
= I started drinking coffee five minutes ago. I am drinking coffee (now). I will finish drinking eventually (in a few minutes).
I have been reading a book for a few days.
= I started reading a few days ago. I may or may not be reading now. I will finish eventually ( in a few days).
I have been studying Spanish for a few months.
= I started studying a few months ago. I may or may not be studying now. I will finish eventually (in a year, in two years, ...).
Past perfect - For a completed action at an unspecified time during a period up to the present (an open time period).
- I have drunk enough coffee. I don't want any more.
= I started drinking coffee. I finished drinking coffee. During an unspecified period up to the present.
- I have read two books this month.
= I started and finished reading two books. This month isn't finished yet. I might read a third book.
Another use of the present perfect - For an ongoing state or repetitive action that has occurred during a period of time up to the present.
- My brother is sick.
= ongoing state
- My brother has been sick for a week.
- It snows in Canada during the winter.
= repeating action
- It has snowed every winter for the last 10 years.