Getting back to the original question (asked 3 years, 8 months ago by someone who's left the group since), the actual answer is that whether you should consider modal auxiliary verbs to be inflected for tense depends entirely on what your grammatical system's definition of "inflected for tense" is, and how your grammatical system uses tense ...
If this is just an exercise, your first clause, using the present perfect continuous, is the correct conversion from active to passive and is grammatically correct:
Active: I have been learning English for 5 years
Passive: English has been being learned [by me] for 5 years
As you noted, it sounds awkward, which can happen with the perfect continuous forms ...
The action was not completed. It has been continuing ..
I have been learning English for 5 years.- present perfect continuous tense
English has been being learned for 5 years by me.
I have learned English. - present perfect tense. The action is completed.
English has been learned by me.
“I just wanted to stop by and see how you are doing”. Would be correct for most cases. In the past or future form. ‘Want’ would be used for present tense.
I assume you mean this in a general sense.
In that case, the fact that you want to see ‘how they are doing’ would remaind the same.
The ‘Tense’ would be more focused on WHEN you are wanting to stop by. ...
In embedded questions—especially when talking and asking about confusing situations—we can have multiple interrogatives in a single sentence.
What we have seen in Chinese can also be easily seen to obtain in
Japanese and English. Take the paradigm (21) in English, for example:
a. Who remembers where we bought what?
b. Who remembers where we