Please help me. How to make a sword that will cause additional damage to certain mobs.

Make a procedure that activates when a mob is hit that id it is a certain mob it deals however much extra damage

that's what they said, make a procedure that activates when a mob is hit and check if it's the specific mob you're looking for and do how much damage you want

It's very simple. Make a global procedure that doesn't need external calling except for when a mob is hit. Just do what everyone else said.

I know how, give me a sec...

Okay, I now how to do it:

1. create your sword
2. create a potion (turn of bottles and status in inventory and HUD) (don't add any triggers to the potion)
3. Add a trigger to your sword that is "when in hand tick" and make it give you the potion effect on that trigger
4. Great a globel trigger that is "entity attacked"
5. do "if source entity has active potion "your potion you created in step 2" 
6. And in the "do" section do "if event/target entity is (sub)type of "the mob you want to take more damage"
7. in the "do" section do "deal "the amount of more damage you want" to event/target entity"

Here's a video: https://drive.google.com/file/d/1BCCL4jkDnn1R3v-Glq6yqCXEhqnpwtEE/view?…

I tested this so I know its works.