Defense Against Open Hands
Most players can defend against riichi to some extent. But a great many players are much looser when defending against calls. This topic will also be covered again in Chapter 8, so for now this page stays at the introductory level.
1. First, Read Whether the Hand Is in Tenpai
There is no point folding if the opponent is not even in tenpai. Unlike riichi, calls do not announce tenpai directly, so you need the skill to read it yourself.
(1) Assume Three Calls Means Tenpai
It is rare for a hand with three open melds to still be noten. Unless the opponent is a complete beginner, once they have made three calls, they are in tenpai more than ninety percent of the time.
(2) If a Honitsu Hand Starts Discarding Its Own Suit, It Is Often in Tenpai
For example, if someone is clearly going for a souzu honitsu and then they start leaving souzu tiles over, you should usually assume tenpai, or at least one-shanten.
(3) If They Suddenly Start Staring Hard at the Pond, They Are in Tenpai
This is very important in real play. If someone who normally barely looks at other players' discards suddenly starts studying the pond, be careful.
Until people get used to the game, they tend to focus only on their own hand until they reach tenpai. Sometimes you can roughly read an opponent's tenpai just from where their eyes are going.
(4) If They Repeatedly Tsumogiri with No Delay, They Are in Tenpai
If someone who normally takes time on every discard suddenly starts cutting immediately, they are probably in tenpai. This is especially suspicious when a player who usually brings the drawn tile back to their hand suddenly checks it once and throws it straight into the pond.
That is the basic idea. If points (3) and (4) feel too difficult, start by watching for (1) and (2).
2. Estimate Whether the Hand Is Cheap or Expensive
For example, if the hand is only open tanyao for 1000 points, there is no need to defend against it at all. Avoid meaningless folding.
The basic clue is dora. If someone has ponned the dora, the hand is absolutely expensive. It is at least mangan. This is obvious, but it matters.
Example 1
Dora 
Suppose the dora is North. Even if South opens like this, you can usually ignore it. It is very likely only open tanyao for 1000 points.
Of course, a hand like this is not impossible:
But even then, if you check which yakuhai have already been discarded, you can usually tell. If North has already appeared in the pond, then it is almost certainly just a cheap 1000-point hand.
Example 2
Dora
Suppose is the dora, and the opponent is the East-round dealer.
Double East plus dora 1 is guaranteed,
so the hand is at least 5800 points.
You should avoid fighting it if at all possible.
Flushes also often become big hands. Especially chinitsu, or honitsu combined with dora, very often reaches mangan or haneman class.
Example 3
Dora
If someone opens like this, it is miserable. The moment you draw a manzu tile, you should think about folding.
Yakuhai are also extremely important.
Example 4
Dora 
Assume this is the South round. That is a frightening set of calls. You can hardly cut anything except genbutsu. Even if it means breaking your own hand apart, you should back off here.
3. Which Tiles Are Actually Dangerous?
If you fully fold every time someone makes a call, mahjong stops being mahjong. Unless the opponent is doing something extreme like calling three yakuhai sets or kanning the dora, you usually do not need to break an entire mentsu just to defend.
The key is simply to narrow down which tiles are truly dangerous.
That part is actually not hard.
Example 5
In a shape like this, the dangerous tile is obvious:
is dangerous.
The probability of daisangen itself is not high,
but once the game has reached the middle stages,
becomes a tile you simply cannot throw.
If it hits, you are dead.
Example 6
Nine times out of ten, this is Toitoi. Tiles that have not been seen at all in the pond are dangerous. Yakuhai in particular are not tiles you should cut.
Since tanki waits are fully possible here, you can treat all honor tiles as dangerous. Especially if the opponent makes a tedashi after three calls, you should read it as either:
- they switched from an East-only tenpai into Toitoi
- or they changed a tanki wait
Example 7
Dora
This looks like an atozuke hand. In that kind of shape, the tile you should mark most carefully is an uncut yakuhai.
Of course, it is also common that the opponent already has a concealed yakuhai triplet. But another common case is a hand like this:
Dora
That is, a partial-shape shanpon built on dora plus yakuhai. If someone is calling from ryanmen shapes on an atozuke hand, you should assume there is a real point-value reason behind it, such as dora dora. If you take it too lightly, it will hurt.
Original Japanese page: http://beginners.biz/mamori/mamori10.html