Leads against 3NT after competitive bidding by Matthias Huberschwiller

A: ♠7 - Your partner has opened with a weak two in Spades, promising a good six card suit and less than ten points.

None of your four-card suits seems promising, therefore you have no reason not to lead your partner's suit.

Your result so far:

A: ♠2 - With a fit, there is no reason not to lead your partner's suit.

You need to lead a small card (not the ace!) in order to indicate the presence of an honour.
Imagine that declarer holds Q-x-x and partner has K-J-10-x-x.

Your result so far:

A: ♦K - You have a lead that permits you to establish four certain tricks in one go!
Therefore, you need to chose it rather than your partner's suit, which declarer has promised to have a stopper in.

Your result so far:

A: ♥J - You have bid your Spades and South has promised to have a good stopper there.
It is unlikely that you will establish your own suit without an outside entry. None of your opponents has bid Hearts, so your partner is likely to have length there. As you have a sequence, you need to lead the Jack.
Your partner may then be able to play Spades through South!

Your result so far:

A: ♥Q - You have two five-card suits; in hearts you have a sequence, but your spade honours are better. How to chose?
South has indicated a good stopper in Spades, this is the lead that he expects.

In Hearts, on the contrary, nobody has bid and even if your opponents have Ace and King, you will never give them an additional trick in the suit by leading it.
On the most lucky days, dummy has the King and your partner the Ace!

Your result so far: