How do I check if a player has died?

toggle3 = true
script.Parent.Touched:Connect(function(hit)
	if	hit.Parent:FindFirstChild('Humanoid')  then
		if hit.Parent:FindFirstChild("Humanoid").Health = 0 then
			local Sword = game.ServerStorage.ClassicSword:Clone()
			toggle3 = false
			Sword.Parent = hit.Parent
end
end)

Error : Workspace.Part.Script:22: 'then' expected near '='

Do not use if ... = ... then use if ... == ... then

Examples of Conditional Statements:

-- 1
if 2 + 3 == 5 then
	print("two plus three is five")
end

-- 2
if 2 + 3 == 5 then print ("2+3==5") end
if 5 > 2 then print("5>2") end
if 2 < 5 then print("2<5") end
if 5 >= 5 then print("5>=5") end
if 5 <= 5 then print("5<=5") end
if 1 ~= 100 then print("1~=100") end
if "some text" then print("It worked") end

-- 3
if 10 > 100 then 
    print("10 is greater than 100") 
else 
    print("10 is less than 100") 
end

-- 4
if 10 > 100 then 
    print("10 is greater than 100") 
elseif 10 > 50 then	
    print("10 is greater than 50") 
elseif 10 < 100 then	
    print("10 is less than 100") 
end

-- 5
local pizzaPlaceIsOpen = false
local tacoPlaceIsOpen = true
local hotdogStandIsOpen = false

if pizzaPlaceIsOpen == true then
    print("Pizza Place")
elseif tacoPlaceIsOpen then  -- var == true is usually unnecessary when var is a boolean
    print("Taco Place")
elseif hotdogStandIsOpen then
    print("Hotdog Stand")
else
    print("Hungry!")
end

You can find this in the roblox wiki.

Here is fixed script:

local toggle3 = true

script.Parent.Touched:Connect(function(hit)
	if hit.Parent:FindFirstChild("Humanoid") and toggle3 == false then -- you forgot to add toggle3 == false
		if hit.Parent:WaitForChild("Humanoid").Health == 0 then -- Check if player health is 0
			local Sword = game.ServerStorage:WaitForChild("ClassicSword"):Clone() -- Added WaitForChild()
			toggle3 = false
			Sword.Parent = hit.Parent -- Put sword in character.
		end
	end
end)

Hope it helped :)

Wiki pages:

Conditional Statements

1st, whats wrong with your script is the "=", if you wanna check if something is equal to a certain value you gotta use "==". if youre having an if statement you gotta use one of these operators , they are very self-explainable but ill be glad to explain. You're script is smart for a begginer like you (if you are), but theres an easier way to check if a humanoid is dead, without check its health, just a simple event.

humanoid.Died:Connect(function()
	-- your beautiful code here
end)

I hope this helped, and i hope you understanded how to use this event! If not, glad to help

and remember, == not =