Question about not-tradeable items in tf2

non tradeable if im right

If it says not tradeable on the item it will never be tradeable.

What's the different between premiun and not? Only entering some servers?

His drops become tradeable when he becomes premium. Some items will stay untradeable (eg achievement items)

Afaik they become tradeable as long they would be tradeable too if he would have gotten them with premium

When you go from a Free to Play to "I purchased something" all items which would have been tradeable for a paid user, become tradeable again. Any items that are untradeable by design, remain that way unless you use Gift wrap. Gift wrap makes untradeable items, tradeable!