Goodman,
In the
relevant reading, where
could is used in the affirmative in an attempt to convey the meaning
succeeded, which is what Belly (and Michael Swan) were talking about,
could cannot be used.
We ran; therefore, we [*could catch / were able to catch / succeeded in catching / caught] the bus.
____________
Compare the following two examples, with different meanings:
-- Were you able to notify the police in time to prevent the crime?
-- Yes, fortunately, I was able to.
(Meaning: Did you succeed in notifying the police ...? Yes, I succeeded.)
-----------------------------
-- Could you notify the police in time to prevent the crime?
-- Yes, I could.
(Meaning: Would you be able (hypothetically) to notify the police ...? Yes, I would.)
("Coded" meaning: Please notify the police .... Yes, I will.)
(Not the same meaning as above.
could cannot be used to convey the "success" meaning above.)
____________
Certainly,
could can be used in other readings, such as
would be able to. This alternate reading is typical in a
so that clause of purpose, examples of which you have quoted above.
CJ