The if statement in Lua is straightforward.
I'll start by giving a very simple example.
You start with the keyword if, a condition,
the keyword then, followed by any block of statements, and
the keyword end.
> i = 0
> if i == 0 then
>> print(1)
>> end
1
It does what you'd expect it to do. I'll show you a more complicated example now.
> if i == 0 then
>> print(1)
>> elseif i > 0 then
>> print(2)
>> else
>> print(3)
>> end
1
You can have an elseif clause, and you can have an else clause.
Both of those are optional.
You could have one without the other, or just have the if clause
by itself.