By spell of illusion one tries to be happy by serving his personal sense which will never make him happy. Instead of satisfying his/her own personal material sense, he/she has to satisfy the sense of the lord.
This system of Yoga called Bhagavadgita was spoken to sun god and sun god explained it to Manu (first king of earth). Over the course of time, it was lost. This time the lord spoke again to Arjuna in the middle of Kurukshetra.

There are five systems of devotions
1) Devotee in passive state
2) Devotee in active state
3) devotee as a friend
4) Devotee as a parent
5) Devotee as a conjugal lover

These stages are called Svarupasiddhi. To understand Bhagavadgita one must have one of these quality.

Lord's personality is difficult to understand. It can not be known even by personalities greater than human. This is why one must read Bhagavad Gita in spirit of devotion.

The subjects of Bhagavad Gita comprehends five basic truths
1) What Gos is 2) What prakriti is 3) What cosmic menifestation is 4) How it is controlled by time 5) What the activities of living entities are

Jivas are small parcels of supreme lord. Therefore Jivas and material nature are part of Prakriti. Jivas are concious. Therefore Jivas prakriti is superior. This does not mean that Jivas prakriti is superior than the God, because lord is concious of all bodies while Jivas (living beings) is concious of his/her particular body.

The lord resides in every living's heart. Lord is the one who gives direction by making determinations. Then according to the actions the living being suffers or enjoys. These karmas can be changed, when the living being is in mode of goodness, in sanity and understands the types he/she should adapt. If this occurs then all of his/her past activities can be changed. Therefore Karmas are not eternal.