
Max M. answered 07/29/19
Harvard Literature major with 20 years of coaching writers
Ok, good question! Simple past, present perfect, and past perfect trip up a lot of people, so you're in very good company.
I'm not quite sure what the "(be)" you've included in your examples mean, but maybe I can straighten things out anyway.
The difference between simple past and present perfect is emphasis.
Simple past, like the name implies, is used to describe things that happened in the past.
Present perfect ("have _______ed" or "has _________ed") also describes things in the past, but emphasizes their impact on the present.
So both "my boss was furious and I was fired" and "my boss was furious and I have been fired" are grammatically correct, but the second one emphasizes how you are now in a state of being fired. In other words, if you used the present perfect, you would be implying that you're still out of work, that you're still upset about it, etc.
Past perfect ("had _______ed") I think of like this: if you're telling a story in the past tense, and you want to describe something that happened before the story you're telling, and which is important to understanding it, you use the past perfect. It's two "jumps" into the past. So, as in your example, "by the time I got to the office (simple past), the meeting had already begun (past perfect)." The story you're telling takes place when you arrive at the office, but you need to describe something that happened before that, so past perfect is the tense that makes that second jump farther back into the past.
Hope this helps!