A partial grammar question letter :

' "Hello Grammar Teacher,

I would like to thank you for your edition. However, I am not able to understand the logic of your edition. Could you..."

Hello Grammar Teacher/Teachers,

At the time of writing the letter, I still  have not understood the logic of the edition.What is...'

Hello Grammar Teacher/Teachers,

1. What is the correct tense or tenses I should or can use in the underlined words ? For instance, is it possible to use a past tense( was not able to understand ) , present perfect tense ( have not understood ) or past perfect tense ( had not understood) ?

2. Is the word ' still ' in bold letters optional ?

Thank you.

With best wishes.