East opens 1NT 5332 shape is good to open with 12-14 HCPs.

South has a difficult decision, whether to overcall or not?

Not sure what has been taught, but it is important to compete against one of the best duplicate contracts of 1NT.

Here you have 9HCPs and a 5431 distribution that is on the left of the 1NT opener which increases the odds that the K♠ s is a trick. You may want to read this short article.

https://www.andrewrobson.co.uk/article/beginner_corner/906

If you bid 2♥ s partner may be tempted to raise to 3♥ s, but it is an overcall of less than 15 HCPs otherwise he would have doubled so North should be cautious.

3♥ s may go off, but as you can see all Easts playing in 1NT made scoring +90 whilst one off in 3♥ s is only +50.

You may see with the little box that with best defence East should go off in 1NT. How?

South starts with the standard lead of 4♥ the fouth in longest suit that makes 5 tricks in ♥ suit. Many people will block the ♥ suit so only 4 tricks will be made at the start. North wins the first trick with the K over the Q. The normal play is to play the top of 2 remaining or the bottom with 3 remaining. So North will make the fourth trick with the 10♥ s.

Then North has to choose what to lead next. How?

You know the opposition has 12-14 + 8 HCPs = 20-22

Add your 10 and you now know partner has 8-10 HCPs.

You must not lead away from your K♦ .or K♣, so lead a ♠ .

This will give your side an additional trick with K♠, he can then cash the remaining 5th ♥  and North will finally make the K♦ ! One off.

West opens 1♣  with their nice hand hoping to rebid 2NT with their 17-18 HCPs.

North overcalls with 1♦ .

South has a choice between 2♦ or 1NT. 1NT shows a stop and the odds are that Souths hand will give one stop.

West rebid his clubs suit as he does not have a hand shape for a double (shortage in opposition suit and support for other suits).

Over 2♣  North has nothing further to say and South will bid 2♦ and whilst West would like to bid he can say nothing more.

South opens longest suit.

North has a choice of which 5 card suit to bid. Generally best to bid the major first.

South does not have the values to bid 1NT (15-16) and not an ideal shape with singleton.

So rebids ♣ s.

North knows they have enough values for game so bids 3♦ s (2♦ s would be weak and could be passed).

Over 3♦ south has to choose between 3NT or supporting ♦ s. 3NT is often easier to make and with a stop in ♥ s bid 3NT is likely to be best.

1♣ opener by North.

East overcalls 1♠ .

South can double with 8+ HCPs and support for other suits.

North only has ♣ s so rebids them.

South shows his stop in the opposition suit by bidding 2NT.

North has a great hand for NTs with the ♣ s - so bids 3NTs.

1♣ by South followed by 1♠ by North.

South rebids his ♣ s.

North knows they have game values and he has support for ♣ s by bidding 3♣ s.

South does not have a stop in ♦ or ♥ s, with both he could bid 3NT. With one of the suits covered he could bid the suit. So he bids 3♠ s, showing 3 of them as he did not support the ♠ bid earlier.

South bids 1♦ .

West can do a weak jump overcall 2♥ s. (if played - if an intermediate jump overcall is used, West has to pass)

North doubles to show support for the other suits and particularly the 4 ♠ s.

South with a beautiful hand bids 4♠ s.

East has a weak 2♦ opener.

South could double, but with 5♠ s and 4♥ s it might be easier to bid ♠ s and if opportunity allows bid or double to show ♥ s.

West can make it difficult for north/south by bidding 4♦ s.  Which will probably be passed out as North with 7HCPs cannot expect game and being vulnerable risks a heavy penalty.