I heard before than in WI you can only defend yourself within the means that you are being attacked. I.E. if someone is attacking you with a knife, you can defend your self with a knife, not with a gun. Also, can you shoot just because you feel your life is in danger? Or must something else happen first? Thanks!
You heard wrong. You can't just shoot because you feel you life is in danger, you have to reasonably believe that you, or another person is in IMMINENT danger of death or great bodily harm. "Imminent" means "about to happen."
I don't know where the "you can't use a gun against a knife" myth keeps coming from. You can use a tank against knife (although you'd probably be using it to help a third party since it would hard to say you reasonably feared great bodily hard from a person with a knife while you're inside a tank.)
A person armed with a knife is capable of causing death or great bodily harm. You can't attack them simply because they are capable, but again, only if you reasonably belief the death or harm is imminent.
A person armed with a baseball bat may also be capable of causing death or great bodily harm.
Some people with no weapons at all are capable of causing death or great bodily harm, particularly if they are much larger or if there is more than one of them. (Disparity of force.)
If the "no gun can be used against a knife" myth was true, it would result in ridiculous consequences, e.g., "Sorry ma'am, I'd like to help you against this murderous rapist who's holding a knife to your throat, but I left my knife at home and all I've got is this here 12 gauge shotgun." or "Yes, I've lost permanent use of my arm because I wasn't allowed to draw my gun on the knife slasher."
As far as your first question-- what are the legal conditions for drawing your weapon on someone-- remember drawing and pointing are two completely different actions. Drawing simply means removing the firearm from the holster. It may be prudent to draw at the first sign of danger. As the saying goes "the fastest draw is the gun already in the hand." I've drawn my gun numerous times in response to different situations without having to actually point it at anyone. For example to investigate a suspicious noise at night.
If I drew my gun without pointing it at someone, but a potential threat is actually there, I'd try to avoid letting them see I have a drawn gun, keeping the gun out of their view. If there's not question that they're a definite threat, for example you've got rioters outside your store, then you may want to let them see you've got a gun in your hand without pointing at them. That may be all it takes to make them go away. And before someone jumps all over me by pointing out you can't use deadly force to protect property, don't forget that not all riots are done with the intention of looting property--- the rioters were not after Reginald Denny's truck, they were after Reginald Denny.
If you meant to say when can you legally POINT your gun at someone, I'd say under the same circumstances when you might be justified to actually shoot them.