This is basic to Dominion. Another example, often misunderstood by new players, is Mining Village.
Let's say you're starting your turn, so you have 1 Action and 1 Buy.
You use your Action to play Mining Village, and resolve it in order, once:
1) Draw a card.
2) Add 2 to your pool of Actions.
3) You now have a choice:
A) Trash the Mining Village and add $2 to your pool of coins.
B) Don't do anything
Let's say you choose A. Now you have 2 Actions, 1 Buy and $2.
Let's instead say you choose B. Now you have 2 Actions and 1 Buy. You use 1 Action to play Smithy. You cannot now trash the Mining Village for $2. You made your choice, you can't change your choice, and you can't go back and resolve any part of Mining Village again. You resolve the card in its entirety, and only once.
Complicating this are cards like Merchant. This came up in a game recently with a new player. "The first time you play a Silver this turn, +$1." The player thought this means that he plays a Silver immediately (in his Action phase) - precisely because you're supposed to resolve the whole card before continuing. But this is a case of setting up a future ability. When you resolve this instruction on Merchant, it tells you to do something in the future. If you Throne Roomed this Merchant, you would resolve this instruction again, so it would tell you to do this in the future one more time. If you then in your Buy phase play a Silver (the first time you do so) you would get +$1 twice.
When it comes to Reactions, they are not active or inactive. After you reveal a Moat, it's still in your hand. That's it. This means you can play it for +2 Actions in your turn, simply because you're allowed to play cards from your hand.
You might also want to take a look at the document in my sig if you want more explanations and answers.