Commenting the why not is key. Half my comments are explaining why I had to use this hack as a warning that the obvious fix doesn't work!

I would argue that if an if statement is hard to parse, replace the entire condition with simpler to read (but way more specific) variables that you assign values (the original condition expression) in the line above. No need for comments in that case

Good advice, just to add to this:

• Comments should be part of code review, having at least two pairs of eyes on comments is crucial. Something that's obvious to one person maybe isn't so obvious to another. Writing good comments is as hard or harder than writing good code, so having it checked for mistakes and quality is a must
• Comments aren't the actual documentation and aren't a reason not to write documentation to go along with your code. Often I see larger projects where each class and function is documented in comments, but the big picture and the how and why of the overall structure is completely missing. Remember that in the real world you often have a lot of folk that need to understand how the code works, who aren't programmers themselves. They can't read the code or don't have access to the code. Writing documentation is still important.
• Please for the love of god when you change code, check if the comments need to be updated as well. Not just around the immediate area, but also the entire file/class and related files. I've worked on large codebases before with a high wtf factor and having the code do something different to or even opposite the comments is a nightmare. I'd rather have no comments than wrong comments.

Don’t comment what a line is doing. Instead, write your code, especially names for variables, constants, classes, functions, methods and so on, so that they produce talking code that needs no comments.

Over and over and over again in my experience this just doesn't work. Readable code does not substitute for comments about what the code should be doing.

Do comment the why

In this day and age of source control I don’t think this is fully necessary. If you want to know the why, you can look into the commit history and see which ticket is connected to it. There you might even see the discussions around the ticket as well. But this requires good source control discipline.

It has helped me many times.