When "could" is used as a modal meaning "possibility in the past", what does its negative form, i.e. "couldn't have done", mean? Is it means something is impossible or something is possibly not true?
For example, " I couldn't have been more than six years old when the accident happened." Does it mean "When the accident happened, it was impossible that I was less than six years old " or "When the accident happened, it was possibly that I was less than six years old"?
The word can be used for either meaning.
"Could" means something is or was possible, and "couldn't" means something is or was impossible, but it's not always a used as literal statement of fact. Sometimes whether or not something is possible is based on the speaker's opinion.
For example, if you say, "He couldn't have committed the crime, because I can prove that he was at work that day," you are saying that you are sure that it was impossible. It's a statement of fact.
In your example, "I couldn't have been more than six years old when the accident happened," the speaker is unsure of the details, but believes they were probably six years old or younger. In their opinion it is unlikely that they were over the age of six.
