Recently I encountered a book written by F. R. Palmer, Modality and the English Modals. I found this book very enligtening. To my surprise, his thorough treatment covers most of my questions on modal verbs in ELL. Palmer knew well what would confuse language learners, even native speakers and took a refreshingly universal approach to address a variety of problem cases on modal verbs. Based on several well-accepted corpuses, his research was strongly supported and compelling.
As I went through this book, I found the following excerpt a bit confusing:
Could and couldn't commonly occur not to indicate past time, but to suggest unreality, usually in what can be seen as an incomplete conditional ('. . . if I wanted to', '. . . if things were otherwise'):
I couldn't do anything like that you see, I mean, I couldn't paint an ordinary sort of portrait.
I know I could be average, but I couldn't be very good and I could never do anything new.
A Gannet† could land and take off easily enough in half the runway.
In all these it would be possible to paraphrase with would/wouldn't be able to. There are, in fact, plenty of examples of would/wouldn't be able to, but with little difference of meaning:
There are season tickets, but you wouldn't be able to commute.
Would Professor Worth be able to sign some cheques this afternoon?
We would be able to have a more rational allocation of resources.
I guess this book is not intended for an ELL so Palmer didn't provide too much context for these examples.
Without context I have difficulty grasping how the proposed protasis ('. . . if I wanted to', '. . . if things were otherwise') relates to them. If I were to reword them, I would just use can/be able to over could/would be able to.
The first one might be the easiest: I'm totally a layman for art; (even if I tried my best,) I couldn't paint a simplest portrait.
Can you please add a few more corresponding contexts to other five examples to show how the proposed protasis would convey the intended meaning?
†The Fairey Gannet was a British carrier-borne aircraft of the post-Second World War era developed for the Royal Navy's Fleet Air Arm.