• A person born in India on or after 26th January 1950 but before 1st July 1987 is a citizen of India by birth, irrespective of the nationality of his/her parents. 
• A person born in India on or after 1st July 1987 is considered as a citizen of India only if either of his/her parents is a citizen of India at the time of birth. 
• Furthermore, those born in India on or after 3rd December 2004 are considered citizens of India only if both of their parents are citizens of India or one of whose parents is a citizen of India and the other is not an illegal migrant at the time of their birth. 
• The children of foreign diplomats posted in India and enemy aliens cannot acquire Indian citizenship by birth. 

Further Reading:

• Citizenship in India
• Difference Between NRI and PIO
• Citizenship Amendment Act 2019 (CAA)