For easier understanding, we will do the explanation of Cit-consciousness first.
Q: Cit kiṁ?
What is consciousness?
A: Jñānasvarūpaḥ.
That which of the nature of knowledge.
The word consciousness here is not having the same meaning as the consciousness we are using parallel to the word "awareness", like body awareness, social awareness etc.; or even when we said somebody is gaining awareness after wake up from coma state.
Let us understand this word "Cit" itself from the sanskrit meaning, because the english word "consciousness" can not really replace its original meaning.
Cit means "knowledge" which is self evident and because of which everything become evident. There is two points which we need to see here. First is "self evident self" and second "making others evident". Self evident me means I don't need any other things/entities to prove my existence. In another word svayamprakāsah/savyamjyotih - self effulgent. I ātmā is self effulgent like "light", when everything in this word is perceivable by other entity because there is light fall on to it, but only light doesn't need another light to make it perceived. Therefore it is self evident, in the same time make others become evident. Now, the fact of light is bhāti-self illumines and all other objects in the word is anubhāti-shined after, we can extend the further to find the ultimate light here.
We can say that the ultimate light of the entire universe is sun. Even in the night, if there is no other local source of light, we can see things at night time because of the moon light, which also reflected light from the sun. Therefore sun is self shining and all other things are shined after (because of which sun light, object become evident to us). But if we analyse further, even though sun illumines the objects, but objects come to evident to us because our eyes are seeing. Therefore because of the present of the eyes sight, the sun (light) is known. The status of the shines is given to the sight now, and the sun become shined after (evident because of the sight). Now our eyes can stare at the object, but the sight takes place only if our mind is there, then mind become shines and sight is shined after. And the last, our mind is functioning only when I ātmā the consciousness as the essence of every being is there to shines. Therefore the ultimate light of everything, because of which everything comes into the acknowledgement is I ātmā the consciousness. I ātmā doesn't need any other light to make me evident, this fact can be answered through the deep sleep experience. Even the mind is not functioning in deep sleep, only ignorance was there, but who is the witness of that ignorance, which after waking up, knowing that there were nothing happened, "I slept well". This self effulgent self is I ātmā the consciousness.
But how can you say Cit as the ultimate light which is self effulgent is knowledge alone? To understand this point, we need to know there are 2 kinds of knowledge:
1. Qualified knowledge (viśesa-jñānam), where an object is distinguished from other knowledge. Example: Pot knowledge, cloth knowledge etc.
2. Unqualified knowledge (nirviśesa-jñānam), knowledge as such (consciousness) which make an object to be known as a qualified knowledge. Example: when any particular object is perceived by our sight, actually we are seeing the light which falls on it. The light per se is the unqualified light, and the light which is fall on to the particular object and perceived by us as the object in our sight is called qualified light.
Every object in this word is nothing but just an specific idea / knowledge. We are taking tree as example. Because the idea of the tree is having roots, trunk, branches and leaves, then even when we see a tree which we have never seen before, we can figure out that it is a tree from the previous tree's knowledge we have. Therefore every object in the world is nothing but an qualified knowledge. When a particular knowledge takes place, it means the unqualified knowledge is manifesting as qualified knowledge. This unqualified knowledge is always there and all pervading, just like the sun light which is always there and pervading all object, taking form as particular object in our sight which we called qualified knowledge.
Because of I-consciousness-unqualified knowledge is always there, therefore all others object come to the being as qualified knowledge. And because of the I, Cit-consciousness is never ceases to be, therefore I is also called Sat-existence.
Q: Sat kim? What is existence? A: Kālatrayē’pi tiṣṭhatīti sat.
That which stays in all three periods of time (past, present, future) is Sat.
Existence means that which remains in 3 period of time (past, present and future). The body is arrived and depart, the though is come and go, but not the consciousness. Consciousness being eternal is call Sat. It was there since time beginning less, being manifest as particular knowledge or even when the particular knowledge is gone, Consciousness is always there.
Question arises here. Why there is no sight of sentiency of a conscious being in dead body or any of the inner things, if I-consciousness being Sat-eternal and all pervading? Is it supposed to have no exception for consciousness to exist if we called it all pervading?
Actually consciousness is exist even in the dead body or inner things, but because of the medium of these particular adjunct (upadhi) can not reflect the existence of Cit. Just like sun light is reflected in a bucket of clear water, but it doesn't reflect in the empty bucket nor in the muddy water. The medium itself doesn't allow the expression of the consciousness, even though it never ceases to be exist.
Consciousness is not limited by time, it always remains in past, present and future. To be called as past or future, anything should be as present first. Therefore 3 period of time can be reduced to 1 present alone. And to know that present is exit, consciousness need to be exist, and need to be exist all the way. So we can see that Cit-consciousness is invariable present in 3 periods of times which is called Sat-existence .
Q: Ānandaḥ kaḥ?
What is happiness?
A: Sukhasvarūpaḥ That which is of the nature of happiness
When consciousness exist in all being (living and non living being) without exception, it is means limitless. There is nothing to limit consciousness.
Why ācārya said the consciousness limitless I is of the nature of happiness? Because happiness is the expression of limitless in this experiential world. By knowing one's own nature is fullness alone, one is happy without keep looking for things to make one self happy. Limitless can not gained by limited things. Only by knowing I am of the nature of fullness, then I can release from this life of becoming.
Evaṁ saccidānandasvarūpaṁ svātmānaṁ vijānīyāt.
In this manner, may you recognised ātmā as yourself which is saccidānandasvarūpam.