Modal Verbs

[Manas Ranjan Mallick]

Some schools of thought treat 'could' as a remote form of 'can' and not as as its past form. The remote form is explained in the following examples:
I couldn't do it. (remoteness of time)
It could happen. (remoteness of possibility)
Could you do me a favor? (remoteness of relationship)
How far this interpretation is correct? In the openion of this forum what is correct?
Another thing which puzzles me is ragarding use of 'should' and 'must'. Here are two sentences.
You must see a doctor.
You should see a doctor.
What is the difference? Are they interchangeable?

[Soup]

Modality vs Tense
This isn't a new idea. The point is that modality is different from tense, that's all:

I couldn't do it.
(remoteness of time)

It could happen.
(remoteness of possibility)

Could you do me a favor?
(remoteness of relationship)

Modals
You must see a doctor.
=> It's required; strongly recommended.

You should see a doctor.
=> It's recommended.