This is very similar to this question but was not satisfactorily answered there.
It's very common for auto-checkers (e.g. Grammarly) to suggest a comma follow 'of course'.
For example, if I write "Of course I do", it underlines the word 'course' and suggests that it be changed to "Of course, I do".
But is that correct? It seems to me that "of course, I do" and "of course I do" could both be correct but mean something slightly different.
In the question linked above, user @HotLicks commented: Basic question: Do you pause anywhere when you say "Of course I don't"?
Possibly that was Socratically leading to the answer but I wanted to try to spell it out.
It seems to me that "Of course, I do" means something like, "on the other hand, this does apply to me." For example, I might say: "If I didn't have a bike, I wouldn't be able to ride to work. Of course, I do so I can."
On the other hand, "Of course I do!" means something like, "I am indignant that you would think that wouldn't apply to me." For example, I might say: "You think I don't have a bike? Of course I do!"
In the above paragraph, 'course' is underlined by Grammarly but the question is:
Is that really an illegitimate usage?