1NT Forcing is an almost essential companion to 2over1 Game Force.

After a major opening (1 or 1), the 1NT response is forcing and promises 5-11 High Card Points with/without support for the opened major.

Opener's rebids (after 1 - 1NTforcing):
2: 3+ cards, 11-16HCP
2: 3+ cards, 11-16HCP
2: 4+ cards, 11-16HCP
2: 6+ cards , 11-16HCP
2NT: 17-18
3: 4+ cards, 18-21 HCP
3: 4+ cards, 18-21 HCP
3: 4+ cards, 18-21 HCP
3: 6 cards, 15-17 HCP
3NT: 19-21 without a void
4: 11+ cards in the two suits, 14+
4: 11+ cards in the two suits, 14+
4: 11+ cards in the two suits, 14+
4: 7+ cards, 15+ HCP

After opening 1, Opener may have to rebid 2 with only a 5 card suit (Hand Shape:4-5-2-2)

Responder's Rebids

Responder's rebids depend on opener's rebids.

Responder can show a long weak suit by bidding it at the lowest level.
Here are some 1N Forcing Rebid Examples after Opener's 1 opening and 2 rebid:

1	1N!
2	Pass: Fewer than 10 points, spade singleton/void, 3+ clubs, no 7 card suit
	2:   a long weak suit without a major fit: 2 32 AJ98765 J32
	2:   a long weak suit without a major fit:  2 AJ98765 32 J32
	2: either two spades, 5-9 HCP: 32 AJ98 432 KJT9       or three card trump support, fewer than 8 points: 432 AJ987 43 JT9
	2N: 10/11 HCP, fewer than 3 trump, invitational        32 AT98 QT8 KJT9
	3:   invitational in opener's second suit, under 11 HCP: 2 A432 A65 J8732
	3:   6+ card fine invitational new suit, under 11 HCP, usually without a stopper in the other major: 2 T32 KQJT87 K87
	3:   6+ card fine invitational new suit, under 11 HCP: 2 KQJT87 T32 K87
	3:   3 trumps, 11 points, invitational: KJ2 QT98 K32 Q87
	3N and above: Consider Using 2/1 Game Force instead of 1Nforcing

Here are some 1N Forcing Rebid Examples after Opener's 1 opening and 2 rebid:

1	1N!
2	Pass: Fewer than 10 points, spade singleton/void, 3+ diamonds, no 7 card suit
	2:   a long weak suit without a major fit:  2 AJ98765 32 J32
	2: either two spades, 5-9 HCP: 32 AJ98 432 KJT9       or three card trump support, fewer than 8 points: 432 AJ987 43 JT9
	2N: 10/11 HCP, fewer than 3 trump, invitational 32 AT98 QT8 KJT9
	3:   6+ card fine invitational new suit, under 11 HCP, usually without the other major stopped: 2 T32 K87 KQJT87
	3:   5+ cards invitational in opener's second suit, under 11 HCP: 2 A432 J8732 A65
	3:   6+ card fine invitational new suit, under 11 HCP: 2 KQJT87 T32 K87
	3:   3 trumps, 11 points, invitational: KJ2 QT98 K32 Q87
	3N and above: Consider using 2/1 Game Force instead of 1Nforcing

After Opener's 1 opening and 2 rebid, both majors are stopped, so 2N can be used for all NT invitations and 3 of a minor can instead be for weak hands of 7+ cards.

1	1N!
2	Pass: Fewer than 10 points, more hearts than spades
	2: either two spades, 5-9 HCP: 32 AJ98 432 KJT9       or three card trump support, fewer than 8 points: 432 AJ987 43 JT9
	2N: 10/11 HCP, fewer than 3 trump, fewer than 4 hearts, invitational 32 A98 QT98 KJT9  or 32 32 K87 KQJT87
	3:   7+ cards, fewer than 4 cards in majors, under 9 HCP, 2 32 873 KQJT987
	3:   7+ cards, fewer than 4 cards in majors, under 9 HCP, 2 32 KQJT987 873
	3:   4+ hearts, invitational: 2 KQT9 T32 QJ7
	3:   3 trumps, 11 points, invitational: KJ2 QT98 K32 Q87
	4:   4+ hearts, 12+ points: 2 KQJT T32 K87

After Opener's 1 opening and 2 rebid, showing 6+ cards, 2N can be used for all NT invitations and 3 of a new suit can instead be for weak hands of 7+ cards. Passing 2 with a void may be logical.

1	1N!
2	Pass: Fewer than 10 points, if short spades no 8 card suit
	2N: 10/11 HCP, fewer than 3 trump, invitational 2 A987 QT98 KJT9  or 32 Q32 K8 KQJT87
	3:   7+ cards, under 9 HCP, 2 543 87 KQJT987
	3:   7+ cards, under 9 HCP, 2 543 KQJT987 87
	3:   7+ cards, under 9 HCP, 2 KQJT987 543 87
	3:   2or3 trumps, 11 points, invitational: K2 QT98 KJ32 Q87
	3N and above: Consider using 2/1 Game Force instead of 1Nforcing

After Opener's 1 opening and 17-18 point 2N rebid, 3 of a new suit can instead be for very weak hands of 7+ cards.

1	1N!
2N	Pass: Fewer than 7.5 points, fewer than 3 spades, no 7 card suit
	3:  7+ cards, under 7 HCP, 3 QJ2 72 Q987654
	3:  7+ cards, under 7 HCP, 3 QJ2 Q987654 72
	3:  7+ cards, under 7 HCP, 3 Q987654 QJ2 72
	3:  3 trumps, 5-6 points, to play: Q32 JT97 Q432 42
	3N: 8-11 HCP, or equivalent, fewer than 3 trumps: 32 Q2 KQ96 J8732
	4:   3+ trump, 7+ points: QT8 JT97 QJT9 42 or Q32 KQT9 QJ43 42

Bergen Raise Supplement for Responder

  When partner opens a major and you have four card support:

bid 3 with an invitational raise (11-12 support points)

bid 3 with a good single raise (constructive: 8-10 support points)

bid 3 preemptively, with less than a poor single raise (0-5 support points)
(Note: Old standard Bergen Raises invert the 3/3 meanings)

Advantages:

extra game/slam tries of 3 (and even 3 if spades are trump) that standard limit double raises don't offer