Contract 3 NT by South - West  leads Queen of hearts.

The key to the solution is the distributions of diamonds.
If the odds are 3-3 the is no problems: it is sufficient to take the leading card with the K ©, savivg the entry to dummy, to play the K ¨ , to go to dummy to cash all diamonds. You could make 3 NT +1.
But if you find diamonds 4-2 as in the diagram not only would you loose the J , but you wouln not cash the last diamonds, and contract will be set one.
Since only 5 diamonds tricks are sufficient to mantain your job, sacrifice one Honour and as your second trick cover your K ¨ with the Ace of dummy, play again Diamonds till you give the Jack: Then you take every return and you can cash the last diamonds, since you have still transportation to dummy with the Ace ©.

Contract 4 © by South - West  leads Ace, King and Queen of ª.

Most answers have been right as far as the refuse of ruffing the third spade was concerned, but many lost themselves half way!
In fact they assumed trumps odds should be 3-3: but odds are 4-2 with the four trumps in East, so South should ruff at dummy with the 8 of trump the eventual continuation spades and overruff if East plays the 9 and not to ruff with the J at dummy, otherwise he would loose a trick in trumps.
Remember: when you are given the possibility to ruff a trick, always consider the advantages to refuse the ruff and discard a loser card instead, expecially whe you are short in trumps, as you are in the example where you are playing 4 © with the Moysian fit 4-3.

Contract 6 © by South - West  leads the King of ª.

Whenever there are shortages in both hands you should consider the possibility of crossruffing that allows you to "long" your trumps and produces a greater number of trump tricks. Pay attention to cash as soon as possible your side winners you do not need in your planning. South will cash A,K ¨, A ª, A § and 8 trump tricks. So, after taking the initial lead with the Ace, South plays little to the Ace § and begin to ruff alternately § and ª.
But, if South forgets to cash immediately his diamond winners, West will be able to discard a diamond on the third round of spades and South, unable to draw trumps, will later loose by a ruff one of his diamond winners

Contract 4 © by South - West  leads the King of ª after he interfered 1 ª.

The declarer should try to create an entry to dummy to cash the last diamond if odds in the suit are 4-2.
So he should take with the Ace, play A,K ©, leaving one opp's master trump at large.Then plays on diamonds discarding a spade loser on the third diamond, ruffs the fourth diamonds in the hand, ruffs a spade at dummy and goes on playing the fifth diamond: East can ruff if he likes but South discards the last spade loser and do his job.
Remember this expedient of not drawing all trumps and leaving one at large. This play preserves an extra trump for declarer, (sometimes in both hands) to be used to gain a trick by a ruff or to create an entry to dummy.

Contract 3 NT by South - West  leads the 4 of ©.

The declarer can make his contract with 4 diamonds tricks, 2 heart tricks, the A ª and A,K §. But to enjoy dummy's diamonds he needs an entry to dummy, so he has to create one playing little heart from dummy and taking in his hand with the Ace!
After that the hand has no more secrets: South plays Ace ¨ and goes on in the suit untill the King wins. The entry at dummy by means of Q,J © , will enable him to cash the remainder of his diamonds.
Remember: stop to think before playing the first card of dummy; very often the future of a contract depends on what is played in the first trick!

Contract 6 ª by South - West  leads the 10 of ¨.

The declarer can count 11 tricks (7 trumps, A ©, A ¨, and A,K §.
The lacking trick could come from the Q ¨ if the King is favourably placed. So he tries to take the leading attack with the Queen, but Est plays the Kind and South wins the trick with the Ace.
The only possibility for South to maintain the contract is the estabibilishment of the fifth heart of dummy, possible if are satisfied both following conditions: hearts must breack 4-3 (p=0,62) and trumps 2-1 (p=0,78) since he needs 3 entries to dummy constituted by dummy's trumps.
South plays then hearts to the Ace and ruffs a heart with the Ace, goes to dummy playing the Queen of trumps to the King, ruffs another heart with the J of spades; plays now the seven of trumps for the 9 of dummy so he can ruff the fourth heart with a high trump.
He may go to dummy playing the 2 of trump covered by the 5 and on the fifth heart may discard a loser diamond.
What is the probability of making the contract after the first trick has been played?
Since both conditions have to be satisfied at the same time (spades 2-1 and hearts 4-3)

the final probability is given by the product of the probability of single events, that is: p=0,78x0,62=0,4836.
Nobody answered right so I couldn't assign the extra 3 points....
But, since I'm very good at heart, I'll give  6 extra points (3 for each questions) to all who tell  me:
1. What is the probability to make 6 S, before East played the K ¨ on the first trick;
2. What is the probability to make 13 tricks before East played the K ¨ on the first trick.
Answers have to be sent to me within Sunday the 28th of January!
Love all