The important thing to remember is that Liquid is a templating language, not a programming language. There are a lot of limitations to the amount of programming logic that you do inside the liquid tags.
The for loop has a few different options, but step is not one of them. If you want to iterate through the integers 2, 4, 6... 20 the only way I can think to do that is to iterate through the values 1 ... 10 and use {% assign val = forloop.index | times: 2 %} inside the loop to get the values that you want.
Similarly, you are not able to perform any operations within an if or unless tag. You are only allowed to make one or more comparisons, connected with either and or or keywords. You cannot even use parentheses or brackets inside your if or unless statements to group your logic. (Parentheses are legal characters, but they're simply ignored) Instead, you will need to assign the value to a variable, then use that variable to make your comparison.
Thanks Dave, good to know the limitations ! Appreciate the concept of multiplying the index of the loop to achieve.