Once plaque has hardened into calculus (tartar), brushing can't remove it. If you think you might brush too hard, hold your toothbrush the same way you hold a pen.
如果你认为刷牙可能太用力，握你的牙刷就想握一支笔一样。
2
The guide way and all gears in the headstock are hardened and precision ground.
在车头的所有齿轮经过高频淬火及精密研磨。
3
The only way we can continue to wear fur is if we have hardened our hearts so that we can no longer feel the pain and suffering that went into our garment.