In your Buy phase, you first play Treasures, then start making purchases: Mint
+1 Action means you add 1 to the number of actions you can play this turn, not that you immediately play an Action: Throne Room on a cantrip.
When you trash multiple cards, you select which cards to trash, then trash them all together: Chapel+Rats
Follow the instructions on the card in order: Wishing Well
If you can't do everything a card says, do what you can: Trading Post (trash 1 card for no benefit if it's all you have in hand).
If it matters, when each player does something, they do it in order starting with the current player. (Spy, or Duchess now Spy is gone.)
The Lose Track rule: Vassal + ...what's simplest?
Duration effects apply even if the card itself leaves play, but while-in-play effects do not: Procession + Lighthouse
While-in-play effects don't apply double when you play the same card twice: Throne Room + Goons
The game only ends if the requisite piles are empty at the end of a player's turn: Ambassador
Pay attention when effects are restricted to specific phases: Peddler + Remodel
A cost of 1 Potion or 1 Debt is neither more nor less than a cost of $1: Chariot Race
The number of cards you've played is not the same as the number of cards you have in play, and if you play a card twice it counts double: Throne Room + Conspirator