в каком случае употребляется Present Perfect, а в каком Past

В каком случае употребляется Present Perfect, а в каком Past Perfect.
a) Look at this concert hall! They ... it recently.
b) They ... most of the cathedrals before others came to their land.
be married
a) By the time their first baby was born, my friends ... for 5 years already.
b) I ... never ... .
know
a) Last week I was introduced to George. I ... him before.
b) Michael is a cool guy. I ... him for ages.
work
a) When my father was promoted to a senior position, he ... at the factory for 20 years.
b) I ... with her since my first day in this company.
leave
a) Mary ... just ... the office.
b) By the time the bell rang, everyone ... already ... .
Задание 2. Поставьте глагол в скобках в правильном медли Present Perfect либо Past Perfect.
Mary and John are such a lovely couple. I ... always ... them. (like)
My aunt is in poor health. She ... in hospital since Monday. (be)
Parents ... to their children before the party, so they behaved quite well. (talk)
I didn't introduce Katie and Ann to each other, they ... already ... before. (meet)
I am going to the library to return this book. I ... it already.