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Lua uses if/then/elseif/else/end. The then is required; end closes the block.

local age = 18
if age < 13 then
    print("child")
elseif age < 20 then
    print("teen")
else
    print("adult")
end

Comparisons return booleans: == ~= < > <= >=. Note ~= (not !=) for inequality.

Logical operators: and, or, not (NOT && || !). They short-circuit and return one of their operands (not necessarily a bool):

local name = user_name or "anonymous"  -- common idiom for defaults

Falsy values: only false and nil. Zero is truthy. Empty string is truthy. Watch out!