when do we use the past perfect?

By a certain time...

My grandparents had died by the time I graduated college.

Internet had already been invented by the year 2000.

By 2010, I had already been to China.

By 2015, I had already quit smoking.

by 2010 ______

by 2005 ________

by the time I graduated university ______

by the time I had my first child _____

by the year 2000 ______

ALIBI GAME AND OTHER PAST PERFECT IDEAS: https://www.tefl.net/elt/ideas/games/fun-past-perfect-practice/