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It's hard for me to understand about past perfect tense. I wrote sentences which the time was messy.

Here are some sentences.
-I had been studying in college for a year and I stopped studying.
-I realized that I did not touch something hot when I had forgotten how to avoid from the fire.
-My friend had met me since I was standing on the sidewalk.
-My family had been cleaning the house for all morning and then a visitor came.
-I had reviewed Physics since the last two hours.
Comments  
None of those should use past perfect. Use past perfect only when the order of two indicated past events is otherwise unclear:

I ate breakfast before she arrived.-- The order of actions is clear, so no past perfect.

I ate breakfast when she arrived.-- Did she see me eating? The order of event is unclear.

I had eaten breakfast when she arrived.-- Use past perfect if she didn't see me eating.
Ah I got it.

I had done my homework when my teacher came in the classroom.
My sister had clean her room since my mother left for work.
Students: We have free audio pronunciation exercises.
Not quite yet.

I had done my homework when my teacher came in the classroom.-- Good.

My sister had cleaned her room since my mother left for work.-- No: the word 'since' not only orders the two events, but you have also used past perfect for the more recent event; past perfect is used to refer to the earlier event.

My sister had cleaned her room by the time her mother came back home from work.
Oh i see. Give me more conjunction words for past perfect used, so I want to know.
Can't think of any offhand. Don't think 'conjunction word'; think about whether the two events are clearly ordered logically or by common sense. If not, use pp.
Students: Are you brave enough to let our tutors analyse your pronunciation?
ok thanks for explaining. Now I understand clearly. Emotion: smile
but it can be: my sister has cleaned her room since my mother left for work.

or should it be has been cleaning.
My sister has cleaned her room since my mother left for work.-- The cleaning is complete.

...has been cleaning...-- The cleaning is continuing now.
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