klemmen
to jam
To no longer be movable, or to be movable only with difficulty; to be stuck or blocked.
Die Schublade klemmt, ich bekomme sie nicht auf.
The drawer is jammed; I can't get it open.

to jam / to pinch
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to jam
To no longer be movable, or to be movable only with difficulty; to be stuck or blocked.
Die Schublade klemmt, ich bekomme sie nicht auf.
The drawer is jammed; I can't get it open.

In this meaning, the thing that is not working is the subject of the sentence. The verb is used intransitively here, meaning without an accusative object.
Der Reißverschluss meiner Jacke klemmt schon wieder.
The zipper on my jacket is jammed again.
When you pinch or trap a body part, a dative construction is used: the person is in the dative (mir, dir), and the body part is in the accusative (den Finger). This shows whom the mishap happens to.
Ich habe mir beim Schließen des Fensters den Finger geklemmt.
I pinched my finger while closing the window.
In this meaning, 'klemmen' (to wedge/clip) always needs an accusative object (what?) and an indication of place or direction (where to?). Typical prepositions are 'unter' (under), 'hinter' (behind), 'zwischen' (between), or 'an' (on/at).
Sie klemmte sich eine Blume ins Haar.
She tucked a flower into her hair.
This meaning is usually used impersonally with 'es' as the subject and is colloquial. You can add the area in which there are problems with 'bei' or with an adverbial phrase.
Bei uns klemmt es gerade mit dem Geld.
Money is tight for us at the moment.
The expression 'sich hinter etwas klemmen' (to get stuck into something) is colloquial and a fixed unit. The 'etwas' is in the accusative and refers to the task one is making an effort on.
Er muss sich hinter seine Hausaufgaben klemmen, wenn er fertig werden will.
He has to get stuck into his homework if he wants to finish.
In the meaning 'stehlen' (to steal), 'klemmen' (to steal) is purely colloquial. You normally would not use it in a formal context or in written language.
Er hat sich im Laden einen Kaugummi geklemmt.
He swiped a piece of gum from the store.
to steal
'stehlen' and 'klemmen' can mean the same thing in a colloquial sense: someone secretly takes something that does not belong to them. 'stehlen' is more neutral and fits almost all situations, including official or serious texts. 'klemmen' in this meaning is colloquial and sounds more casual, sometimes also a bit cheeky. Important: 'klemmen' has many other meanings, for example when a door is stuck. If you are talking about a real theft and want to be clear, 'stehlen' is the safe choice.

In der Schule hat jemand meinen Stift geklemmt.
At school, someone pinched my pen.
Warum stiehlst du Geld aus der gemeinsamen Kasse?
Why are you stealing money from the shared cash box?