Okay.... As you are new I will give you the polite heads up as some others will not. Nightfalls are a once a week reward. You can play them again and again but you will only receive blue engrams, motes and the occasional legendary engram after your first run. You can use three of coins if you like for an exotic on top of your first nightfall reward. You can also just receive strange coins and 3 of coins for completing your first run. Raids work in the same way. If you do normal mode you will only get rewards once per weekly run through on each character. If you do hard mode after then you will receive hard mode rewards for that run through on each character. However if you do hard mode first you can receive both normal and hard mode drops in the one raid. That then becomes your full raid completion and no further drops will occur apart from some bosses who drop exotics with 3 of coins.