Rules of Bridge (I believe Bolded rules more important)

Rule of 1
When there is just 1 trump out higher than yours, it is normally best to leave it out. To eliminate their top trump costs you two trumps and gives up the initiative, possibly neither of which you can afford.

Rule of 2
When missing 2 non‑touching honors, and have less than 9 of the suit, it is usually superior to deep finesse; i.e., to first finesse for the lower missing one and then again for the higher. With 9 pieces, you should only use the simple finesse.

Rule of 4
Avoid giving partner 3-card support for his 5‑card Major suit if a likely 4‑4 fit is available in the other Major. The 5-3 Major side suit can later be used to throw off a losing trick and you usually will make one more trick with the 4-4 suit as Trump than you would have with the 5-3 suit as Trump in that hand.

Rule of 7
Declarer use at NT contract when have only 1 stopper. Count suit (yours & dummy), subtract from 7, hold up that number of leads if can.

Rule of 8
With 8 cards in a suit, including the Ace, King, and Jack, it is normally best to finesse for the queen on the second round after playing the Ace. Similarly for the Jack if holding the King and Queen and ten, after playing the King. This is to eliminate the loss to a possible singleton honor.

Rule of 9
With 9 cards including the Ace, King and Jack, it is normally better to play the two top honors hoping to drop the Queen; i.e., do not finesse on the second round. Similarly for the Jack when holding the King, Queen and ten of the suit. “Nine never, Eight ever”.

Rule of 9
Pass partners take out double, if the number of cards you hold in the opponent's suit + honors you hold in that suit (including the 10) + the level of the contract >=9.

Rule of 11
Use when the opening lead is the fourth best from the defender's suit. 11-lead spot = number of cards higher than lead spot.

Rule of 19/20
Count your high cards, add the length of your two longest suits if you reach 19/20 INCLUDING 2 quick tricks, open. Rule of 19 used in England, USA uses rule of 20.

Rule of 15
In 4th seat, if high cards, + number of spades = 15+, open.

Rule of 17
Partner bids a weak two.  If high card points + your cards in partner's suit = 17, try for game. Conversely with at least 2 trump, if have 2 1/2 quick tricks, bid 3.  With 3 1/2 quick tricks, bid 4.

Rule of 23
If you know your side has at least 23 HCPs and opponents “have the bid,” you have only two options. Double or bid on. Don't Pass!

Rule of 20
First & second position. Add HCPs When considering opening the bidding in first or second position, statistically speaking, your hand is strong enough to open if you can count 20 or more (HCP’s , plus one point for every card you hold in the two longest suits); and is not strong enough to open if the count is less than 20.

Rule of 30
When partner reveals a void and you have a strong trump fit (or solid suit), there are only 30 relevant points, not 40. Six may then be bid on 23‑24 points, and Seven is possible at 26.

Rule of 40
Your points + Dummy's high-card points + any shown on the lead, and any from the bidding.  Deduct total from 40. You'll have a good idea where the other points are likely to be placed.