If

If[condposneg]
returns pos if cond evaluates to True, and neg if it evaluates to False.
If[condposnegother]
returns other if cond evaluates to neither True nor False.
If[condpos]
returns Null if cond evaluates to False.

If the second branch is not specified, Null is taken:

You might use comments (inside (* and *)) to make the branches of If more readable: