On the one hand they'd love to have kids, but on the other, they don't want to give up their freedom.
一方面，他們想要孩子，但另一方面，他們又不想放棄自由自在的生活。
2.
On the one hand, if the body doesn't have enough cholesterol, we would not be able to survive. On the other hand, if the body has too much cholesterol, the excess begins to line the arteries.
on the one hand是正確表達
例句
1.
On the one hand they'd love to have kids, but on the other, they don't want to give up their freedom.
一方面，他們想要孩子，但另一方面，他們又不想放棄自由自在的生活。
2.
On the one hand, if the body doesn't have enough cholesterol, we would not be able to survive. On the other hand, if the body has too much cholesterol, the excess begins to line the arteries.
一方面，如果體內沒有足夠的膽固醇，我們就無法存活下去。另一方面，如果體內膽固醇過高，多餘的膽固醇就會附著在動脈內壁上。
3.
On the one hand I admire his gifts, but on the other I distrust his judgment.
一方面我羨慕他的才華，而另一方面我卻懷疑他的判斷力。