If your attack succeeds, you deal damage. The type of weapon used determines the amount of damage you deal.
Damage reduces a target's current Hit Points.
In 2-handed mode if the weapon has a Versatile property the new damage will override the base damage dice. ( Longsword, Battleaxe, Quarterstaff ) In off-hand mode the ability score modifier is not applied to the damage. It's also possible to enable the 'Two-weapon fighting style' to ignore the previous rule. Two handed really is much better though. #1 Great Weapon Master -5/+10 heavily skews the damage math in its favor. #2 If you have any other Bonus Actions to spam from class or feats, that's where TWF really falls off, you either give up half your damage for the turn or never use those features, where great weapon can do full damage and use them. Two Weapon Fighting – This fighting style (also commonly referred to TWF) allows you to add your ability modifier to the damage of your offhand attack, increasing your damage by up to +5! Only Fighters and Rangers get this style by default, but anyone with a martial weapon proficiency can get it through the Fighting Initiate feat.
Normal: If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. When fighting in this way you suffer a –6 penalty with your regular attack or attacks with your primary hand and a –10 penalty to the attack with your off hand. If your off-hand weapon is light, the penalties are reduced by 2 each. Two light or one-handed main and light off-hand: -4/-8 TWF, with proper Two-Weapon Fighting feat (requires 15 Dex): Two one-handed weapons: -4/-4. Two light or 1h & light: -2/-2. Double-bladed sword's secondary attack automatically counts as a light weapon. As you can see, it's best to wield two light weapons, or a one-hander in the main hand.
Minimum Damage: If penalties reduce the damage result to less than 1, a hit still deals 1 point of Nonlethal Damage.
: When you hit with a melee or thrown weapon, including a sling, add your Strength modifier to the damage result. A StrengthPenalty, but not a bonus, applies on damage rolls made with a bow that is not a composite bow.
Off-Hand Weapon: When you deal damage with a weapon in your off hand, you add only 1/2 your Strength bonus. If you have a StrengthPenalty, the entire Penalty applies.
Wielding a Weapon Two-Handed: When you deal damage with a weapon that you are wielding two-handed, you add 1-1/2 times your Strength bonus (Strength penalties are not multiplied). You don't get this higher Strength bonus, however, when using a light weapon with two hands.
Multiplying Damage: Sometimes you multiply damage by some factor, such as on a critical hit. Roll the damage (with all modifiers) multiple times and total the results.
Note: When you multiply damage more than once, each multiplier works off the original, unmultiplied damage. So if you are asked to double the damage twice, the end result is three times the normal damage.
Exception: Extra damage dice over and above a weapon's normal damage are never multiplied.
Offhand Dmg Two Weapon Fighting Pathfinder 2e
Ability Damage: Certain creatures and magical effects can cause temporary or permanent ability damage (a reduction to an ability score).