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Present perfect tense is used when:
1. an action is finished and this action has a connection to the present
I have opened the window. (means the window is still open now)
I have done my assignments. (means my assignments are still 'completed', the action of doing assignments is finished and its connection to the present is the state of the assignments is still finished)
2. a situation that started before and lasts up till the present
It has been a sunny day. (means the sun is still shining now)
The weather has been cold. (means it is still cold now)
3. with words 'since' and 'for'
I have started learning piano since I was ten years old.
I have started learning piano for 5 years.
Both sentences imply 'I am still learning piano.'
Past perfect is used when:
1. an action is finished, and this action is related to another action in the past; we use past perfect to show an earlier action and simple past to show a later action
I had done my assignments when my mum arrived home.
2. an action is finished and its effect does not last up till the present
I had lived in Paris. (means I am not living in Paris now but I was living in Paris before)
3. with words 'since' and 'for'
I had started learning piano since I was ten years old.
I had started learning piano for 5 years.
Both sentences imply 'I am not learning piano now'.