1. Tongue
We all know we cannot speak without the tongue. So the tongue is closely related to speech. To hold one's tongue (保持沉默) means "to keep silent". A person who has too much tongue (太多嘴) is disliked by all, for he is too talkative. Mother tongue is not the tongue of a mother: it is a person's native language.
"Don't you have a mouth below your nose (你鼻子底下不是有张嘴吗)?" The Chinese say so to blame a person who did not say what he should have said. But this is not the right way to express the idea in English. English people would say, "You have a tongue in your head, haven't you?"
2. Face
"Face" has to do with the idea of respect and dignity both in Chinese and English. You lose your face (丢面子) if you fail again and again, but a decisive victory will save your face (挽回面子) after all your failures.
When you feel unhappy, you pull a long face (拉长脸). The idea is conveyed in Chinese in the same way. But "about face (向后转)" does not refer to the face. It is a military order to turn round and face in the opposite direction. It is the exact equivalent of "about turn".
3. Shoulder
The shoulders can bear heavy things. Your father has a great responsibility for the family on his shoulder. When faced with difficulty, the family should stand shoulder to shoulder (肩并肩) to overcome it. You should not turn a cold shoulder (不理睬) to your family members. Nor should you give your friends the cold shoulder (冷落朋友). Both expressions mean treating others coldly.
4. Back
A good host is hospitable to his guests. But, if a guest is tiresome, the host is glad to see his back (他离开).
If your friend has done something really well, you may give him a pat on the back (轻轻拍背部) to show your appreciation.
If you turn your back on somebody (不理睬他人), you mean you do not like making friends with that person.
5. Heart
The heart is an extremely important organ inside the chest. It usually stands for something important or the centre of something, for example, the heart of the matter (问题的核心) or the heart of mystery.
Many other expressions make use of the word "heart". A lover can be called a sweet heart (情人). The Purple Heart (紫心徽章) is a medal given as an honor to American soldiers wounded in a battle.
If your friend is in trouble, you may encourage him by saying "Don't lose heart (不要灰心)."
6. Stomach
The stomach is naturally related to one's appetite. If you dislike heavy food, you have no stomach for it (反胃). The word is also related to one's interests or likings. If you find something boring or vulgar, you have no stomach for it, either. Bad food turns your stomach. Similarly, your stomach turns at a bad joke.
Stomach can also be a verb. Look at this example: "How could you stomach (忍受) such rude words?" Apparently, stomach here can be replaced by "tolerate".
7. Arm
Every person has two arms: the right arm and the left arm. The right arm is usually stronger, so we call a good helper the right arm (得力助手).
We all know an arm is not very long. But when you keep someone at arm's length (保持距离), the distance is long enough. For that means you do not like that person and you try your best to avoid him or her. We should indeed keep the bad friends at arm's length.