einstecken
to put in
To put something into a pocket, bag, or another container.
Er hat sein Portemonnaie in die Jackentasche eingesteckt.
He put his wallet in his jacket pocket.

to put in / to take
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to put in
To put something into a pocket, bag, or another container.
Er hat sein Portemonnaie in die Jackentasche eingesteckt.
He put his wallet in his jacket pocket.

'Einstecken' (to put in / to take) is a separable verb. In a main clause, the prefix 'ein-' comes at the end of the sentence. In the infinitive with 'zu', 'zu' comes between the prefix and the verb.
Er steckt den Schlüssel in seine Hosentasche ein.
He puts the key in his trouser pocket.
The verb consists of 'ein-' (in/into) and 'stecken' (to put/stick). The literal meaning 'to put something into something' is the basis for the figurative meanings.
Steck den Stecker in die Steckdose ein.
Plug the plug into the socket.
You can recognize the literal meaning from the fact that it involves a concrete object (e.g., phone, money) and a concrete place (e.g., pocket, backpack).
Sie hat den Brief schnell in ihre Handtasche eingesteckt.
She quickly put the letter in her handbag.
In the figurative meaning 'to endure,' 'einstecken' is often combined with negative, abstract things. Typical phrases are: 'Kritik einstecken' (to take criticism), 'eine Niederlage einstecken' (to suffer a defeat), or 'einen Verlust einstecken' (to take a loss).
Die Mannschaft musste eine schwere Niederlage einstecken.
The team had to suffer a heavy defeat.
The meaning 'to secretly keep something for oneself' is colloquial and has a negative connotation. It implies that the action is dishonest.
Hat er das Wechselgeld einfach eingesteckt?
Did he simply pocket the change?
to stick / to be stuck
'stecken' and 'einstecken' can both mean putting something into something else. 'stecken' is more general: you can put something into a hole, into an opening, or between other things. 'einstecken' fits especially well when you put something into a pocket, bag, or container and then take it with you. The words are easy to confuse because 'einstecken' looks like a form of 'stecken'. When it is about a jacket pocket, trouser pocket, or handbag, 'einstecken' often sounds more natural.

Vor dem Termin hat Anna ihr Handy schnell eingesteckt.
Before the appointment, Anna quickly put her phone in her pocket.
Kannst du den Schlüssel in das Schloss stecken?
Can you put the key in the lock?