Which of These Actions Is Forbidden by the Constitution?

The answer: creating an ex post facto law

Among common answer choices, the action forbidden by the Constitution is passing a law ex post facto. The Constitution explicitly prohibits this in two places: Article I, Section 9 forbids Congress from passing ex post facto laws, and Article I, Section 10 forbids the states from doing so. "Ex post facto" is Latin for "after the fact."

An ex post facto law is one that reaches backward in time to punish. It does one of three things: it criminalizes conduct that was legal when it was done, increases the punishment for a crime after it was committed, or changes the rules of evidence to make conviction easier after the fact. All are banned because they are fundamentally unfair, you cannot obey a law that did not exist when you acted.

Why the distractors are permitted

On this style of question the wrong answers are usually powers the government legitimately holds, which is why they are not forbidden:

• Collecting taxes is an enumerated power of Congress under Article I, Section 8. It is expressly permitted, not forbidden.
• Declaring war is likewise granted to Congress in Article I, Section 8.
• Regulating commerce between the states is another explicit Article I, Section 8 power.
• Punishing crimes in general is permitted; only retroactive punishment through an ex post facto law is barred.

The trick is recognizing the difference between an exercise of granted power (allowed) and an action the Constitution specifically places off-limits (forbidden). Ex post facto laws fall into the second category alongside a short, memorable list of prohibitions.

The bigger picture: the full list of forbidden acts

Students score better when they can recite the Constitution's explicit prohibitions rather than guessing. The main ones are:

• Ex post facto laws (Art. I, Secs. 9 and 10): retroactive criminal laws, barred for both Congress and the states.
• Bills of attainder (Art. I, Secs. 9 and 10): laws that declare a person or group guilty and impose punishment without a trial. Also barred for both.
• Suspending the writ of habeas corpus (Art. I, Sec. 9): forbidden except in cases of rebellion or invasion when public safety requires it.
• Titles of nobility (Art. I, Secs. 9 and 10): the government may not grant them.
• Taxes on exports (Art. I, Sec. 9): Congress may not tax goods exported from any state.

Section 10 adds state-only prohibitions: states may not coin money, enter treaties, impair the obligation of contracts, or keep troops in peacetime without congressional consent.

Notice that ex post facto laws and bills of attainder appear in both Section 9 (limits on Congress) and Section 10 (limits on states), a signal of how seriously the framers took protection against retroactive and trial-free punishment. Knowing which branch each limit targets, and that these two apply to everyone, turns a guessing question into a certainty.