Thank you for your correction but I have been complicating with a sentences that has double past while I taught in school that the past verb should not followed by another past verb. For exampe I had understood (had and understood is past).
What you are doing with the verb HAVE is to make it into past tense ( HAD) and "conjugate" it with "understand" in order to express the past period of that action.
Conjugation of 'Understand'
Base Form: Understand
Past Simple: Understood
Past Participle: Understood
3rd Person Singular: Understands
I understand. (Naelewa)
I understood. (Nimeelewa)
He understands. (Anaelewa)
He understood. (Ameelewa)
I had understood. (Nimeshaelewa)
Please try to study the 12 verb tenses in order to understand your grammar formation better.
Once you have mastered the tenses, you will automatically know how to conjugate the verbs through the tense formulas.
Here is the example of Past Perfect , one of the tenses.
Form:
[had + past participle]
Examples:
You had studied English before you moved to New York.
1. Here the past participle of study is studied.
2. Then you just put into the formula and it becomes "had studied".
Had you studied English before you moved to New York?
USE 1 Completed Action Before Something in the Past
The Past Perfect expresses the idea that something occurred before another action in the past.
It can also show that something happened before a specific time in the past.
Examples:
I had never seen such a beautiful beach before I went to Kauai.
I did not have any money because I had lost my wallet.
We were not able to get a hotel room because we had not booked in advance.
A: Had you ever visited the U.S. before your trip in 2006?
B: Yes, I had been to the U.S. once before.
USE 2 Duration Before Something in the Past (Non-Continuous Verbs)
With Non-Continuous Verbs and some non-continuous uses of Mixed Verbs, we use the Past Perfect to show that something started in the past and continued up until another action in the past.
Examples:
We had had that car for ten years before it broke down.
By the time Alex finished his studies, he had been in London for over eight years.