39. what is a ranged spell attack? it's mentioned in the firebolt description, among many other attack spells.
Attacks break down in a couple of ways: Ranged versus melee, and weapon versus spell. You can have ranged weapon attacks, melee weapon attacks, ranged spell attacks, and melee spell attacks.
Ranged versus melee: Ranged attacks are made at disadvantage if there is an active enemy within 5 feet. Melee attacks do not suffer this penalty. Casters often overlook the fact that this rule applies to
all ranged attacks, weapon or spell.
Weapon versus spell: Weapon attacks use Dexterity or Strength as the attack stat
1, depending on the weapon; and you add the stat bonus to both attack and damage rolls
2. Spell attacks use your spellcasting ability as the attack stat
3; and you add the stat bonus to the attack roll but
not the damage roll
4.
1. Unless otherwise specified, e.g., hexblade warlocks.
2. Unless otherwise specified, e.g., dual wielding without the fighting style.
3. Unless otherwise specified, e.g... uh... can't think of an exception to this one.
4. Unless otherwise specified, e.g., warlock using Agonizing Blast.
Side note: There is also some confusing terminology around "melee" and "ranged." An
attack with a melee weapon is different from a
melee weapon attack. For example, throwing a spear is an
attack with a melee weapon (because the spear is on the Melee Weapons list) but it is a
ranged weapon attack (can hit from a distance and suffers disadvantage if an enemy is close). This is why I play spellcasters.
40. is it possible to make more than one attack with an off-hand weapon (while dual-wielding)?
The short answer: No. No matter how many attacks you make with the main hand, you only ever get a single off-hand attack.
The long answer: 5E doesn't really have "off hands," per se. If you have a weapon in each hand, and you take the Attack action with one of them, you can make a single bonus-action attack with the other. You're free to switch up which weapon you use for the regular attack and which for the bonus action. But the bonus action only ever gives you one attack.
41. is the dueling fighting style usable while holding a shield in one's off-hand?
Yes. In fact, that is the normal way to use that fighting style. It's the "sword and board" option.
42. does the druid's nature's ward class feature give immunity to both poison damage and the poisoned condition?
Yes. "Immune to poison" covers both.