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A partial letter:

' Hello Grammar Teacher,

I would like to thank you for your edition.However, I cannot understand/ have not understood/have not been understanding the logic of your edition.Could you ...'

Hello Grammar Teacher/Teachers,

1. At the time of writing the letter, I still have not understood the logic of the edition.What is the correct tense or tenses such as a present or past tense I should or can use in the underlined words in order to communicate with my Grammar Teacher ?

2. Could you tell me whether the following words can be used in order to replace the words in the underlined words of the letter or not ?

a. have yet to understand

b. am not able to understand

c. could not understand

Thank you.

With best wishes.
Comments  
It is best to replace all those words underlined with those in item b. About tenses, I have read an article TENSES which may be of help to you.
Hello Grammar Teacher,

I would like to thank you for your advice. I have accepted item b.,that is, am not able to understand.However, I am doubtful about the rest.

Could you tell me whether those underlined words,including item a.(have yet to understand ) can be accepted as alternatives in that situation ?

Further, I would like to thank you for recommending the article on TENSES. I have read them and they are easy to comprehend but they do not

show us when to use them.

Thank you.

With best wishes.
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I would like to thank you for your edition.However, I cannot understand/ have not understood/have not been understanding the logic of your edition.Could you ...' b. am not able to understand

I was reading the thread and wanted to offer a little direction. I sawa that were given a link about tenses of verbs. Let me offer another:

"In English, there are three basic tenses : present, past, and future. Each has a perfect form , indicating completed action; each has a progressive form , indicating ongoing action; and each has a perfect progressive form , indicating ongoing action that will be completed at some definite time." - http://leo.stcloudstate.edu/grammar/tenses.html

You indicated that you picked "letter b" as the one you will use. There is only one small problem with that: to say, " I am not able to understand" implies that you have some kind of mental deficiency. It is better to say:

a. I did not understand. . . Perfect form - past -or-

b. I do not understand. . . Progressive form - present
In any written communication, clarity is very important. Items a and c are not suitable.

Item a (have yet to understand) means or implies that you will understand in due course even without aid, while item c (could not understand) is a past situation without indicating the present state and may imply that you can understand now.

The article on TENSES is actually a blog post which may not be suitable for elaborating their uses.

You may see useful links in another member's reply.
"Not able to understand" does not necessarily imply that the person has some kind of mental deficiency which could be just one of the (I would say, unlikely) causes of the inability. The inability could be due to the edition being unclear.
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