[lamford]

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Camrose (Home Internationals in UK, a strong event), Wales v Northern Ireland.

Partner leads the ace of diamonds. You are East. Plan the defence and give reasons!

[rhm]

Declarer can not have five hearts and six clubs, because this would give partner too many HCP in spades and diamonds and anyway partner does not hold seven good diamonds for this bidding.
So declarer is likely balanced with around 19 HCP with 3 diamonds and four hearts.
Declarer is either 3♠=4♥=3♦=3♣ or 2♠=4♥=3♦=4♣.
This means declarer has one diamond ruff, 4 club tricks, 4 trump tricks and a spade trick.
We probably have 2 diamond and a spade trick.

One possibility is to play us for 2 spade tricks.
This would require declarer to have

♠Kx ♥AKTx ♦QJx ♣AQxx or ♠Kxx ♥AKTx ♦QJx ♣AQx

However, this defense would hardly be tough.

More likely we do not have a second spade trick, in which case the fourth trick can only come from trumps.
This will require partner to have the heart ten and the diamond queen and declarers spade trick must be slow. That requires partner to have the spade ace.

Give declarer ♠KQ ♥AKxx ♦Jxx ♣AQxx or ♠KQx ♥AKxx ♦Jxx ♣AQx and we can force him
Dump your diamond king and partner can play 3 rounds of diamonds forcing dummy to ruff.
Partner will play another diamond when he comes in with the ace of spades.
If partner has the ten of hearts we have a trump trick or declarer draws trumps immediately and never gets a spade trick.

Rainer Herrmann

[lamford]

rhm, on 2017-March-06, 05:13, said:

Give declarer ♠KQ ♥AKxx ♦Jxx ♣AQxx or ♠KQx ♥AKxx ♦Jxx ♣AQx and we can force him

That was declarer's hand and your defence works, but was not found at any table. Well done again, Rainer!