I know the difference between can use and may use, and the difference between use and possess, but i just saw the preview for legion legionand part of his trait says can use battle fury i cant remember any other time is specifically says use battle fury but does that mean he can choose to use it or not?

Can use means that you cannot choose.

If a character had a special power that said they can use P/PB and TK they can use them when they are given the appropriate action

However, in the case of passive powers, can use means cannot not use because you do not have to do anything to use the power

And, before anyone posts that I am wrong, please read what I actually wrote, not what you might think I did

The Suit of Sorrows is a good example of "can use" Battle Fury.

"Can use" means "uses". I'm not positive, but I don't think there are any passive powers that have "may use". If there was, it would specify a duration. Something along the lines of "When this character makes an attack, it may use Battle Fury for that attack".

Absolutely nothing is optional unless you give a character an action to use it. The action is the option.

Edit: 'may' indicates something within an action is an option

Actually, quite a few passive powers give the player the option of using the power. Energy Explosion, Super Senses, and Shape Change a few examples of these.

Sort of. Those powers contain "may" in their wording but they are always on. The power needs to trigger for you to get to the text containing the "may" that allows you to decide whether or not to use the effect. I was talking more about "may use" in place of "can use" as in "may use Shape Change".

Now that there is the PG entry which states anything triggering off a "use" of a power needs for an action to be given or its special effect to be utilized if it is passive, this is somewhat of a moot point.

No, I agree with that, I was more addressing the "Absolutely nothing is optional unless you give a character an action to use it."