das Knie
knee
The joint in the middle of the leg that connects the thigh with the lower leg.
Er hat sich beim Fußballspielen das Knie verletzt.
He injured his knee while playing soccer.

knee
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knee
The joint in the middle of the leg that connects the thigh with the lower leg.
Er hat sich beim Fußballspielen das Knie verletzt.
He injured his knee while playing soccer.

You can easily recognize the meaning of 'Knie' (knee) from the context. If it is about the body, sports, or medicine, it refers to the joint. If it is about a quick decision, the idiom 'etwas übers Knie brechen' (to rush something) is meant. If it is about defeating an opponent, 'jemanden in die Knie zwingen' (to bring someone to their knees) is meant.
Beim Wandern schmerzte mein linkes Knie.
My left knee hurt while hiking.
Pay attention to the case after prepositions. Movement is expressed with the accusative, while a position or place is expressed with the dative.
Das Kind fiel auf die Knie.
The child fell to its knees.
The idiom 'etwas übers Knie brechen' (to rush something) is used as a fixed expression. The thing that is decided hastily appears as the accusative object.
Man sollte eine Heirat niemals übers Knie brechen.
One should never rush into marriage.
In the idiom 'jemanden in die Knie zwingen' (to bring someone to their knees), the person or thing that is defeated is the accusative object.
Die hohen Kosten haben die Firma in die Knie gezwungen.
The high costs brought the company to its knees.
joint
'Knie' and 'Gelenk' both have to do with movement in the body. But the 'Knie' is a specific body part on the leg. A 'Gelenk' is more general: the shoulder, elbow, finger, and hip are also joints. So you can say: the knee is a joint, but not every joint is a knee. The words can be confused when talking about pain or injuries. If you mean the exact spot on the leg, 'Knie' fits. If you are talking generally about movable connections in the body, 'Gelenk' is better.

Nach dem Sturz im Garten tut mir das Knie weh.
After the fall in the garden, my knee hurts.
Die Ärztin untersucht das Gelenk, weil Paul den Arm kaum bewegen kann.
The doctor examines the joint because Paul can hardly move his arm.