Pinfu
Pinfu is one of the easiest yaku to build.
If you organize your hand efficiently, the hand will often turn into Pinfu naturally.
1. Improving into Ryanmen
Pinfu must be made entirely of sequences, and the final wait must be a ryanmen wait.
That means the key to building Pinfu is to keep creating ryanmen shapes.
Example 1
Example 1 is also a review from Chapter 1.
Following the rule of thumb that kanchan is better than penchan,
you should discard .
If the pinzu shape draws 
, it becomes a ryanmen shape and Pinfu is guaranteed.
If it draws 
, it turns into a ryan-kan shape and the acceptance widens.
A penchan shape does not have this kind of improvement.
Example 2
Example 2 compares two kanchan shapes.
Here too, you should compare which one is more likely to improve into ryanmen.
The manzu kanchan improves into ryanmen if you draw either or .
The pinzu kanchan improves only when you draw .
So you should break the pinzu kanchan instead.
That means this hand starts by discarding .
Tsumo
However, if you later draw ,
then the right discard becomes .
That is why you must start by cutting .
2. Ryan-Kan
This is also a review from Chapter 1.
Ryan-kan means a shape where two kanchan gaps are connected,
such as 
.
Whether you draw or , you can make a sequence.
That is why it is considered the best shape after a true ryanmen.
Using this well makes Pinfu easier to build.
Example 3
Tsumo
In this hand, the correct discard is ,
which builds a ryan-kan shape in pinzu.
Discarding gives the same number of accepting tiles,
but discarding makes Pinfu more likely, so it is better in terms of point potential.
3. Dropping a Pair
If your head is a value honor tile, the hand cannot be Pinfu.
You have to be careful about that.
Example 4
Tsumo
Even though it is a value honor, the normal play here is to drop the pair of .
This is a hand being played with Tanyao and Pinfu in mind.
It is not the kind of hand where you should call and settle for a cheap 1000-point win.
It is a hand where you aim for a high-value closed hand.
This kind of pair-drop to pursue Pinfu is very common, so remember it.
Example 5
Tsumo
Example 5 is a famous problem.
Discarding and discarding are not the same.
If you cut , the manzu shape becomes
,
which also accepts .
This kind of composite shape, ryanmen plus kanchan, appears often enough that you should not overlook it.
Example 6
Tsumo
If you discard in Example 6, your sense for the shape is a little off.
Here you should drop the pair of instead.
If you draw , you become one-shanten for Pinfu;
and if you draw or , you can still continue toward Pinfu.
It may look like dropping the pair of is also possible,
but then the only tile that improves into ryanmen is .
Another option is to float one ,
but I do not recommend it, because it weakens your discard flow and makes it harder to keep a safe tile in hand.
Original Japanese page: http://beginners.biz/teyaku/teyaku04.html