首先,楼主一个单词打错了,估计是应该focus吧,还有teenager这里要用单数
It is hard to focus on being happy when you are a teenager.
focus on sth.专注于某事；集中
这是一个由it作形式主语的主语从句,为了避免头重脚轻而把真正主语置后,意思上等于： To facus on being happy when you are a teenagers is hard.当你还是青少年的时候很难专注于一直保持开心.