Hi,
I think you can use a comma to sort of link two separately written, yet very related-in-content clauses like this:
Thank you, it was good to receive a gift of appreciation by you last week.
I think 'Thank you' and what follows can be thought of as two separate clauses yet can be linked by a comma.
That was his decision, deciding to support his care his aged father by himself and not put the father somewhere difficult to care for was the right thing to do. -- Since the underlined clause contains an element, which is 'decision'. that is further elucidated by the longer clause that followed, I think it could be argued a comma is correclty used. Right?
But you wouldn't agree this use of a comma is correct, would you?
That was thrown abruptly, he could not have anticipated its coming while quite immersed in his conversation with his old friend. -- 'it' refers to a ball, in this case.
Are there a guideline as to this use of comma (if it is in fact correctly used)?