if-else is similar to other languages. The boolean condition needs to be surrounded by parentheses and each condition is followed by a block.
if-else conditionals are expressions.
/mob/Login() var/n = 5 if (n < 0) world << "[n] is negative" else if (n > 0) world << "[n] is positive" else world << "[n] is zero"
You can also construct if statements on a single line, as such:
if (n == 7) world.log << "Special number!"
Using the code in the first block, try adding a condition to check if the number is divisible by two.