How to make an effect that deals damage every 10 seconds but not kill entity? (like poison)

this is the simplest way to do this. if you want it to deal 3 damage, then change the number 0 to the damage you want -1.

get the health of thw entity and if its at your threshold then don't deal damage els deal damage put all this in a wait statement

heres some pseudo-code :- 

wait 200 ticks:

        if not {[get health of 'event/target entity '] == 1}:

                 deal damage of type 'generic'

note that there is a NOT there before teh condition

And you're English ant that bad ;)

in fact its pretty good