One of his upper front teeth was loose.

他的一颗上门牙松动了。

Now, you make the th sound with the lips relaxed open, and the tip of the tongue lightly touching the area just behind the back of your upper front teeth.

发 th 音的时候嘴唇放松张开，舌尖轻轻接触上门牙后面的区域。

His face was yellow and drawn and his upper front teeth showed slightly—he, too, seemed agitated as far as it was possible for his well-balanced temperament to be so.

他的脸色发黄， 皱巴巴的，上门牙微微露出来——他似乎也很焦躁， 因为他那平衡的气质可能会如此。

For example, how much would you have to be paid to have one upper front tooth pulled out? Or how much would you have to be paid to have one little toe cut off?

比如，给你多少钱你才愿意拔掉自己的一颗门牙，或者给你多少钱你才愿意砍掉一根小脚趾？

He treated a syphilitic child and pointed out to Mr. Singer the scaling eruption on the palms of the hand, the dull, opaque surface of the eye, the sloping upper front incisors.

他治疗了一个患有梅毒的孩子， 并向辛格先生指出了手掌上的鳞屑疹、暗淡、不透明的眼睛表面， 以及倾斜的上前门牙。

With an f consonant sound, as in 'fork', and 'from' and 'fry' the lower lip touches the underside of the upper front teeth but loosely, so there's a little bit of space through which the breath travels.

以 f 辅音发音，如“fork”，“from”和“fry”，下唇接触上门牙的下侧，但很松，所以有一点空间让呼吸通过。