How do you know which ability modifier to use when attacking?
When you attack you roll a D20 and add modifiers. What exactly is the modifier? Well I know, for example a longbow would be D20+Dex modifier. But how do I know which stat is for which weapon? Like, would a short-sword be strength or dexterity?
here's a link to the current (9/2015) version of the basic rules xanderh mentions
Attacks with ranged weapons usually use your DEX modifier.
Attacks with melee weapons usually use your STR modifier.
Weapons with the finesse property use either your STR or your DEX modifier -- your choice.
Weapons with the thrown property use the same stat as they would when not thrown (that is, usually STR, unless it is also a finesse weapon, in which case you choose between STR and DEX)
In all cases, you also add your proficiency bonus (+2 for a level 1 character, and increasing occasionally as you level up) if you are attacking with a weapon that you are proficient with. Your character class (and occasionally race or feats) will tell you which types of weapons you are proficient with.
(see PHB p. 149 for a weapon chart that shows which weapons have the finesse or thrown property. See PHB p. 14 under weapons for which modifier to use)
@Bash thanks, I updated the answer to change "always" in the first statement to "usually", and removed the "melee" qualifications for the finesse and thrown rules. I believe this is fully accurate now.
To-hit is always strength unless otherwise noted.
It's not intuitive but there is a reason. There are a fixed number of stats and you really don't want too much "ability" to rely on a single stat.
Strength is your attack and athletics stat, dexterity is your physical reaction, dodge and acrobatics stat.
If all your attack and defense relied on one stat then everyone would have 20 of that one stat
It's really just of function of this.
Unless stated otherwise:
- Ranged weapons use Dexterity
- Melee weapons use Strength
Weapons with the finesse property use either Dexterity or Strength (your choice).
Might want to add: Melee weapons with the 'thrown' property still count as melee weapons. There might be some spells or class features which allow you to use other ability modifiers instead or additionally. (i.e. Paladin's 'Sacred Weapon' *(PHB p.86)*)