TH asked: When a responder changes suit, it is forcing on opener to rebid. However what happens if opener’s rebid changes the suit again? Is this forcing on responder to bid again?
An opener's change to a new suit that is not a jump bid or higher ranking than their first bid (eg 1H - 1S - 2C) is not normally forcing when the responder has changed suit at the one level. It may indicate a two-suited minimum or medium range opening hand (13-18 points) and a responder with minimum (6-7) points may pass with a better fit in the second suit.
If opener changes the suit with a jump bid (eg 1H - 1S - 3C) then this is generally considered game forcing showing 19+ as well as the second suit.
If opener bids a new suit higher ranking that their first bid (commonly called a "reverse" bid) this generally shows at least 17 points (eg 1C - 1S - 2H), and most play it as forcing, but even if not strictly forcing a responder would rarely pass to protect partner with a 19-21 point hand.
If the responder has bid a new suit at the two level (showing at least 10 or 11) then any new suit by opener (eg 1S - 2C - 2H) is generally regarding as forcing to protect an opener with a 15+ point hand.
TH also asked: What if opener’s bid is passed by responder, but opener rebids and changes the suit. Is this forcing on responder to bid?
It is not normally forcing unless opener bids the opponent's suit. However it does show a stronger or more distributional hand and so responder may now choose to bid to compete with the opponents. Opener can also double after an opponent overcall and partner's pass as a way of showing a stronger hand that wants partner to contribute to the decision making.
