Article 20(1) of the Indian Constitution prohibits Ex Post Facto laws. The expression Ex Post Facto Law means a law, which imposes penalties or convictions on the acts already done and increases the penalty for such acts. In other words, Ex Post Facto Law, imposes penalties retrospectively. For example, The Dowry Prohibition Act, 1961 came into force from

20.5.1961. A person guilty of accepting dowry is punishable under the Act after 20.5.1961 and not before 20.5.1961.
Ex post facto laws are of three kinds as follows:(a) A law which declared some act or omission as an offence for the first time after the completion of that act or omission. (b) A law which enhances the punishment or penalty for an offence subsequent to the commission of that offence. (c) A law which prescribes a new and different procedure for the prosecution of an offence subsequent to the commission of that offence.

Clause (1) of Art. 20 provides protection only in respect of the above first two categories of expost facto laws i.e. laws which declare acts as offences subsequent to the commission to those acts and laws which enhance the penalty subsequently.
Article 20(1) provides: No person shall be convicted of any offence except for violation of a law in force at the time of the commission of the act charged as an offence, nor be subjected to a penalty greater than that which might have been inflicted under the law in force at the time of the commission of the offence. The first part of clause (1) provides that no person shall be convicted of any offence except for violation of law in force at the time of the commission of the act charged as an offence. The second part of clause (1) protects a person form a penalty greater than that which he might have been subjected to at the time of the commission of the offence.