"(1) No person shall be punished for an act or omission which was not punishable by law when it was done or made, and no person shall suffer greater punishment for an offence than was prescribed by law at the time it was committed.
(2) A person who has been convicted or acquitted of an offence shall not be tried again for the same offence except where the conviction or acquittal has been quashed and a retrial ordered by a court superior to that by which he was convicted or acquitted."
Article 11 of the Constitution

Under the Constitution, you cannot be punished for anything you do that is not illegal at the time when you did it. As an example, if you roller-blade along Orchard Road today when it is permitted, you cannot be punished 1 month later when the law is changed.

Further, the law cannot give a more severe punishment for a crime committed than that prescribed by the law at the time when the crime was committed. For example, if at present the maximum fine for spitting in public is S$200.00 and you were caught by the police today, then if the law is changed tomorrow and the maximum fine increased to S$2,000.00, you can only be fined up to S$200.00 and not S$2,000.00.