How I can print this? :

Using both of these people's answers this is the least complicated way to do this in a Local Script:

local plyr = game:GetService("Players").LocalPlayer

plyr.CharacterAdded:connect(function(char)
    local h = char:WaitForChild("Humanoid")
    h.Changed:connect()
        if h.Health < h.MaxHealth then
            print"Health Reduced"
        end
    end
end

Hope it helps!(BTW I was the first one to say "Hope it helps!" dsboy stole from me! :O)

You'll need to use a whileloop. This will test if the players' health has less than the first number. Here's an example. This script would be in a localScriptinside StarterGui.

while wait() do
oldhealth=game.Players.LocalPlayer.Character.Humanoid.Health
if game.Players.LocalPlayer.Character.Humanoid.Health<oldhealth then
print ("Health Reduced")
end
end

Hope this helped! -ds

In order to do this, we need to let the script know each time the player takes damage. We'll do this with the HealthChanged() event:

game.Players.PlayerAdded:connect(function(Plr)
    game.Players.CharacterAdded:connect(function()
        local Humanoid=Plr.Character:FindFirstChild("Humanoid")
        local Health=100 --Sets previous health
        Humanoid.HealthChanged:connect(function(NewHealth) --Fires when health changes
            if NewHealth<Health then --Checks if health is higher or lower than before
                print("Health Reduced!")
            end
            Health=NewHealth --Sets the previous health to the new health.
        end)
    end)
end)

Whenever the players health changes, this checks if it's lower or higher than the previous health, and if it's lower, it prints "Health Reduced!"

Anyways, hope this helped you a bit. if you have any further questions/problems, please leave a comment below. Hope I helped :P