If I play a harbinger card, I draw from my deck and then have an action and am able to take a card from my discard and put it onto my deck. Am I able to draw a card, then put one from my discard onto the top of my deck and then play an action card that allows me to draw that card right off the top and play it or do I have to wait until the very end of my turn to move the card to my deck?
Short answer: yes, you can draw it immediately with, e.g., another action card.
Long answer: the golden rule in Dominion is to follow a card's text entirely, top to bottom, before moving on to your next play. In this case, you:
- Play the Harbinger.
- Draw a card from your deck, shuffling it if empty.
- May optionally put a card from your discard pile on top of your deck, if there are any cards in your discard.
- Play subsequently another action card, such as a Village, to immediately draw that card.