The 5 Topics or Subjects or tattvas of the Bhagavad-gita

"The subject of the Bhagavad-gītā entails the comprehension of five basic truths. First of all the science of God is explained, and then the constitutional position of the living entities, jīvas. There is īśvara, which means the controller, and there are jīvas, the living entities which are controlled. 

If a living entity says that he is not controlled but that he is free, then he is insane. The living being is controlled in every respect, at least in his conditioned life. 

So in the Bhagavad-gītā the subject matter deals with the īśvara, the supreme controller, and the jīvas, the controlled living entities. Prakṛti (material nature) and time (the duration of existence of the whole universe or the manifestation of material nature) and karma (activity) are also discussed. The cosmic manifestation is full of different activities. All living entities are engaged in different activities. From Bhagavad-gītā we must learn what God is, what the living entities are, what prakṛti is, what the cosmic manifestation is, how it is controlled by time, and what the activities of the living entities are." From Introduction to Bhagavad-gita As It Is

Īśvara-tattva - the supreme controller

ahaṁ sarvasya prabhavo mattaḥ sarvaṁ pravartate... Translation: "I am the source of all spiritual and material worlds. Everything emanates from Me." Bhagavad Gita 10.8

Jīva-tattva - the controlled living entities

mamaivāṁśo jīva-loke jīva-bhūtaḥ sanātanaḥ... Translation: "The living entities in this conditioned world are My eternal fragmental parts." Bhagavad Gita 15.7

Prakṛti-tattva - material nature

mayādhyakṣeṇa prakṛtiḥ sūyate sa-carācaram... Translation: "This material nature, which is one of My energies, is working under My direction, O son of Kuntī..." Bhagavad Gita 9.10

Kala-tattva - time

kālo 'smi loka-kṣaya-kṛt pravṛddho... Translation: "Time I am, the great destroyer of the worlds..." Bhagavad Gita 11.32

Karma-tattva - activity

karmaṇy evādhikāras te mā phaleṣu kadācana... Translation: "You have a right to perform your prescribed duty, but you are not entitled to the fruits of action." Bhagavad Gita 2.47

See also: The 10 Topics or Subjects or tattvas of the Srimad Bhagavatam