Responder's 1NT to an opening major is a 1-round force.
Playing 2/1, the bid can have as many as 12- points, and can show a variety of hands.
Opener rebids her suit with 6+, or shows a 2nd suit (4+ hearts, or 3+ minors). S/he jumps to the next level with these shapes and 16+ points.
With a weak hand, the forcing nt bidder either
(a) passes with a minimum
(b) bids another suit at the two level to show a weak hand and suit preference,
(c) bids partner's suit at the 2 level to show a preference for that suit (holding 2 or 3 trump support, depending upon partnership agreement -- Hardy's and Lawrence's 2/1 system show 2, with a direct raise showing 3),
(d) bids partner's suit at the 3 level as a limit raise showing exactly 3-card support,
(e) bids 2nt to show 10-12 hcp and a balanced hand inviting 3nt,
(f) signs off in game, but showing 3 card trump support..
You should work out the responses with your partner.
Forcing NoTrump typically only applies where partner has opened and responder is NOT a passed hand and there is NO interference. In such cases 1nt reverts back to its standard meaning. Some partnerships play NT after a passes hand as semi-forcing. asking opener to continue only with a strong hand; others play the NT as forcing in all hands.
One modified form of the Forcing NT is to use the 2C response by opener as a negative bid, showing a hand with a minimum and no other four card suit (2D therefore promises at least 4 cards).
S: xx
H: KQJxxx
D: x
C: xxx
S: AQJxx
H: xx
D: KQx
C: Qxx
W N E S
1S
P 1N P 2C
P 2H P/O
Notes: This sequence shows a long suit in a weak hand.
source of sample hand: bbuff@user.rose.com