what is the best when HARDFORK happen, we have bitcoin or we sell bitcoin? 
It largely depends on your priorities and preferences. If you value your fiat more than you value your crypto, it may be worth selling under the assumption that the price will drop further. But if you value your Bitcoin more than your fiat, it's probably safer to hodl in case you can't buy it back cheaper at a later point. Ultimately it's a choice you'll have to make for yourself, because no one knows for certain how it's going to play out and any choice has its potential pitfalls.
But yes, if you do decide to hodl, do it somewhere safe where you have full control over the keys. Don't leave your coins in a webwallet or exchange, because there's no guarantee they'll honour the coins on your preferred fork.
