I'm pretty sure that I can use 'past tense' where we can use 'past perfect'. The only difference that I feel is

Question

I'm pretty sure that I can use 'past tense' where we can use 'past perfect'. The only difference that I feel is if speaker is willing to emphasize the fact something had happened before another happened. Here are examples.

1. They started to work on this project, and about a month after that, I started.

2. They had started to work on this project, and about a month after that, I started.

3. They started to work on this project about a month before I started.

4. They had started to work on this project about a month before I started.

I feel all of them are available and natural, I prefer #2,4 though. Am I right?

Answer 1

In your examples maybe the difference is not obvious but suppose these sentences:

1) When I got there they went.

2) When I got there they had gone.

Sentence #1 could mean when you reached the place, you observed that they are leaving the place.

Sentence #2 could mean when you reached the place, you didn't see them. they had left there.

In my opinion past perfect has a time sequence within. It says the activity finished in a certain point in the past. that point could be the start point of another activity in the past.

-----past perfect--------}
                          ----- past -------- } present

But there is no order between two past activities

-----past -----------------------past --------}
--------------past ------------- past ------- } present