What is the difference between past perfect tense and present perfect tense? What is the difference between "I had completed my work" and "I have completed my work"?
Past is completed action and no presence of now.
Past is specific time, but present could be now or starts or continues.
I live in Hong Kong over 3 years now, It does mean I have been living in Hong Kong over 3 years, now. ( You can not write I am living in Hong Kong over 3 years now as present continuous tense is short term)
I lived in Hong kong for 3 years, I am now not living there.
Present perfect tense is something starts and it has presence till now, no specific anything more or less.
I have traveled to Manchester three times, but you don't know specifically-- when.
I traveled to Manchester three times last year. ( specific)
Past perfect tense has happened in an unspecific time but completed action. And it usually happens by two facts not one fact, please keep in mind, accordingly, Raymond Murphy.
So I had a cow, it means a possession issue of past tense of "have" no past perfect tense issue, keep in mind!
"I had completed my work before my teacher asked for them", it is the completed action by two facts in the past, no present any links, clear?--- not one clause for past perfect tense.
But I have completed my work --means, I finished my work in an unspecific time but the presence of its impact is till now but we don't know when specifically.
I make sure you have clearly understood the difference both of them.
However, you have to understand the present indefinite tense more too as it has vast explanations. Unless you understand 86 types of tense usages by Raymond Murphy, an examiner and writer to Cambridge English, I doubt you will be easily confident so that my suggestion is to read them all one by one to be a confusion free learner in tense issues.