var foo = 1, bar = 0;
if (foo)
{
    bar ++; foo = 0; 
}

Is stacking code on the same line allowed? Please expand on the code formatting section on the Jam page it's not very clear.

Yes, please also answer to the question, whether we have to use brackets for one lining if statements.

Something like:

if(keyboard_check(ord("W")) then y-=2;

if(keyboard_check(ord("W"))
{
   y-=2;
}

Declaring variables in one line is fine. Whenever you have to put a semicolon you have to make a new line. In Gamemaker you don't need to put semicolons but you SHOULD! Separate statements need to be on different lines. Comments also don't count as lines of code because the game isn't executing anything. Thanks for the question and sorry we were not clear on that earlier.