About leading Aces: Just don’t lead an Ace if you don’t have the King. Such an Ace is called an unsupported Ace. The most common error in bridge is getting overly anxious to cash an Ace when you don’t have the King, especially early in the hand.
That said, there is a worse error.
Never underlead an Ace in a suit contract on opening lead
Note that does not say “try not to” or “prefer not to”. There is a weird case where it is right. It comes up once every five years, and you’re better off not knowing it. Never means never.
So, you can lead the Ace without the King if the only alternative would be to lead a low one in a suit contract. In notrump it is not only ok it is common to underlead an Ace. You’re investing in some long-suit tricks later. In a suit contract, those tricks are unlikely to cash.
Later in the hand, this rule does not apply. It is sometimes right to underlead an Ace. For example, you need to get the lead in your partner’s hand and believe he might have the King.
So, back to leading unsupported Aces. Just don’t do it.
Don’t do it even if your side bid the suit. I swear to God the King will be in the dummy and the declarer will be void. He ruffs it, gets to the dummy, and pitches a loser on the King. On another day, Declarer has Qxx and you just gave him a trick later. Don’t lead that Ace and you will find Declarer has the King or Queen and you’ll capture it with your Ace later.
Aces are meant to capture honors, not small cards.
The time to cash an Ace is when it is “trick taking time”. I regret there is no big neon sign announcing that it is trick-taking time, but when you see or know that the declarer is ready to run a long suit, and will be able toss the losers in your Ace’s suit, the bell rings and you start cashing.
In a slam contract it can be right to cash an Ace on opening lead. It can be horribly wrong. Don’t ask me for help. They don’t have TWO fast losers in your suit. Listen to the bidding. If it sounds like dummy has a long, strong suit, maybe this is the time to cash that outside Ace first.
Cash the setting trick, disregarding all other rules.