A fairly basic question but it would be great to have a definitive answer and I can't find one anywhere.

What is the correct format for boolean criteria for a conditional expression (like an if, or to select form data)? 

And does it matter which format you use? (Because it doesn't seem to make any difference when I test out the different versions).

A. One equals, e.g.

1. FormData = Form[bool = true]

or e.g.

1. if (bool = true) {<expression>;}

B. Two equals, e.g. 

1. FormData = Form[bool == false]

or e.g. 

1. if (bool == true) {<expression>;}

C. No equals, e.g. (I assume this only works for 'if true')

1. FormData = [bool]

1. if (bool) {<expression>;}

Much appreciated,

Katherine