In breathing mindfulness ,why only long and short breaths are taken into consideration?

Why normal breathing situation is ignored during breathing mindfulness?

Normal breathing happens automatically,you are not aware of it!

Watch your breath!

The secret lies in the simple observance and bringing awareness and presence to the breath.This is one of the main teachings of the Buddha! The process of breathing in and out is not a parallel movement as one might think.

Between the in breath (down) and the out breath (up) and again between the out and in breaths there are gaps.Fear of losing identity causes a jump over the gap because "I" or "me" doesn’t exist there.

As I have learnt in S.N. Goenka's Vipassana course, you have to be aware of normal breath only. If you are not able to concentrate on breath in normal mode only then you make a forced breathing to be long or short.

So the normal is norm.

I can guarantee that you can take normal breathing. And you should take normal breath. Without trying to make it long or short.

While breathing in long, he knows “I am breathing in long”, or, while breathing out long, he knows “I am breathing out long”; or, while breathing in short, he knows “I am breathing in short”, or, while breathing out short, he knows “I am breathing out short”.

As you state, Normal breathing is not ignored but it just a way to teach meditators.

The goal of this sentence is you should know every single moment of breath.

If the mind has right mindfulness (which is a quiet mind due to keeping the mind free from craving), it will naturally & automatically become aware of the breathing. This being aware of the breathing (with mindful equinimity) will cause the breath to calm. In this calming process, the breathing will become longer. However, when stored internal stress is touched, the breathing will become short until the stress is calmed & dissolved. Therefore in the proper practise of Anapanasati, the breathing will be both long & short (even though the calming process inclines to long breathing). For example, a normal long in-breath causes the abdomen to expand. However, an extra long very calm & refined in-breath will first cause the abdomen to slightly expand but then mostly contract.

The first two (preliminary) stages Anapanasati are merely descriptions of what the mind should be aware of when it has right mindfulness. To know a breath that is long and to know a breath that is short shows the mind has discernment, clarity, sensitivity & right mindfulness.

MANY KINDS OF BREATH

While we practice following, we have the opportunity to observe various characteristics of the breath. We can feel many things, For example, we can feel the longness and the shortness of the breath. We naturally learn about the long breath and short breath. Then we observe the coarseness and fineness of the breath. Further, we observe its smoothness and burnpiness. Later, we will observe the [physical & mental] reaction to these qualities. In this first step contemplate these different kinds of breath: long and short, coarse and fine, easy and uneasy. Begin to observe them by feeling them with sati. (51)

We must learn how to observe in more detail, that is, to observe the reaction or influence of the different kinds of breathing. What reactions do they cause, how do they influence our awareness? For example, when the breathing is long, how does it influence our awareness. What reactions does the short breathing cause? What are the influences of coarse and fine breathing, comfortable and uncomfortable breathing? We observe the different types of breath and their different influences until we can distinguish clearly how the long and short breaths, coarse and fine breaths, and comfortable and uncomfortable breaths differ . We must know the variations in the reactions to and influences of these various properties of the breath, of these qualities that influence our awareness, our sensitivi­ty, our mind. (52)

Along with the above observations, we need to watch the effect or flavor of the different kinds of breath. The flavors that arise are kinds of feelings, such as, happiness, non-happiness, dukkha, annoyance, and contentment. Observe and experience the flavors or effects caused, especially, by the long breath and short breath, by the coarse breath and fine breath, and by the easy breath and uneasy breath. Find out how it is they have different flavors. For instance, we will see that the long breath gives a greater sense of peace and well being, it has a happier taste than the short breath. Different kinds of breath bring different kinds of happiness. We learn to analyze and distinguish the different flavors that come with the different kinds of breath that we have scrutinized. (53)

Finally, we will discover the various causes that make the breath either long or short. We gradually will find this out for and by ourselves. What causes the breathing to be long? What kind of mood makes the breath long? What kind of mood makes it short? Thus, we come to know the causes and conditions that make the breath long or short. (54)

Anapanasati