I am aware of the usage of COULD in the following cases (list from Cambridge Grammar (https://dictionary.cambridge.org/grammar/british-grammar/modals-and-modality/could)):
- possibility in the present and the future.
- suggestions and permissions
- past ability
- past single events with verbs of senses and mental processes
Also, this makes clear that sentences like "I could visit you yesterday but I was lazy" or "He could travel to China a week ago" are incorrect.
But then the are sentences like:
- Hundreds of years ago, even a simple injury could be fatal.
- In the ancient Rome, you could have both male and female lovers and nobody wondered.
For the following examples, I was told that these are statements of fact:
- I could buy it yesterday and I did.
- Back then, travelling could be dangerous.
What usage of "could"would that be? I cannot see a fitting case in the overview. Why those are correct even when "COULD" refer to the past events?