das Gesicht
face
The front part of the human head, which includes the eyes, nose, and mouth.
Er wusch sich am Morgen das Gesicht mit kaltem Wasser.
In the morning, he washed his face with cold water.

face
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face
The front part of the human head, which includes the eyes, nose, and mouth.
Er wusch sich am Morgen das Gesicht mit kaltem Wasser.
In the morning, he washed his face with cold water.

The word consists of the prefix 'Ge-' and the stem 'Sicht' (sight/view). The prefix 'Ge-' often forms collective terms. So you can think of 'Gesicht' (face) as the whole of what one 'sees' in the front part of the head.
Sie hat ein sehr freundliches Gesicht.
She has a very friendly face.
When a person performs an action on their own face, German often uses a reflexive dative pronoun. The structure is: subject + verb + sich (dative) + das Gesicht (accusative).
Nach dem Sport wäscht er sich das Gesicht.
After exercising, he washes his face.
The meaning 'Gesichtsausdruck' (facial expression) is often recognizable from verbs like 'machen' (to make) or 'ziehen' (to pull/make). Actions like 'waschen' (to wash) or 'schminken' (to put makeup on) refer instead to the physical part of the head.
Als er den Preis sah, machte er ein langes Gesicht.
When he saw the price, he made a long face.
The idiom 'das Gesicht verlieren' (to lose face) is a fixed unit and means to lose standing or respect. The opposite is 'sein Gesicht wahren' (to save face).
Er wollte die Diskussion nicht fortsetzen, um sein Gesicht zu wahren.
He did not want to continue the discussion in order to save face.
The expression 'jemandem etwas ins Gesicht sagen' (to say something to someone's face) follows a fixed pattern: the person you are speaking to is in the dative, and the information is in the accusative.
Sie hat ihm endlich ihre Meinung direkt ins Gesicht gesagt.
She finally told him her opinion directly to his face.
head
'der Kopf' and 'das Gesicht' both belong to the upper part of the body. 'Der Kopf' means the whole part with the brain, hair, ears, face, and skull. 'Das Gesicht' is only the front part of the head with the eyes, nose, and mouth. Learners may confuse the words because the face is part of the head. For headaches, hair, or bumping into something, you are more likely to say 'Kopf'. When talking about appearance, eyes, nose, mouth, or facial expressions, you say 'Gesicht'.

Nach der Arbeit war ihr Gesicht ganz rot von der Kälte.
After work, her face was completely red from the cold.
Simon hat in der Pause Kopfschmerzen und legt den Kopf auf den Tisch.
During the break, Simon has a headache and rests his head on the table.