How to gain weight for a male hardgainers age 18, weigh 50kg and what is the cause?

Glad you work out: The fact that you know the word "hard gainer" tells me you're already doing resistance training. I trust you have good protein intake. Creatine and an electronic muscle stimulator (till it hurts, if your physician is okay with it) will help you. Hard-gainers seem genetically favored for health later in life and most women like the body that lifting gives you more than a "huge" guy. Stick with it!