reinstecken
to stick in
To stick something into an opening, a container, or something else.
Hast du den Brief schon in den Umschlag reingesteckt?
Have you already stuck the letter into the envelope?

to stick in / put in
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to stick in
To stick something into an opening, a container, or something else.
Hast du den Brief schon in den Umschlag reingesteckt?
Have you already stuck the letter into the envelope?

'Reinstecken' (to stick in/put in) is a separable verb with the prefix 'rein-'. In a main clause, the prefix goes at the end; in a subordinate clause, the verb stays together.
Er steckt den Schlüssel in das Schloss rein.
He sticks the key into the lock.
The verb 'reinstecken' (to stick in/put in) is considered colloquial. The prefix 'rein-' is a short, informal form of 'hinein-' (into/in).
Steck das Geld einfach in deine Tasche rein.
Just stick the money into your pocket.
You can recognize the meaning of the verb from the object. With concrete objects (key, letter), the literal meaning is intended. With abstract nouns (time, money, effort), the figurative meaning 'investieren' (to invest) is intended.
Sie hat den USB-Stick in den Laptop reingesteckt.
She stuck the USB drive into the laptop.
In the figurative meaning, 'reinstecken' (to put into/invest) is often used with abstract nouns such as 'Arbeit' (work), 'Mühe' (effort), 'Zeit' (time), 'Energie' (energy), or 'Geld' (money).
Wir haben viel Energie in die Renovierung der Wohnung reingesteckt.
We put a lot of energy into renovating the apartment.
to insert / to begin / to use
'einsetzen' and 'reinstecken' can both mean bringing something into something else. 'einsetzen' sounds more precise and fits when something goes into the correct place and is supposed to sit there or function there, for example a battery or a window. 'reinstecken' is more colloquial and more general: you simply stick something into a bag, opening, or box. There is also some overlap with time, money, or effort: 'Ressourcen einsetzen' (to use resources) sounds matter-of-fact, while 'viel Arbeit reinstecken' (to put a lot of work into it) sounds more everyday and colloquial. The words can be confused because both express putting something in or using something.

Tom steckt den Brief schnell in seine Tasche rein.
Tom quickly sticks the letter into his bag.
Der Techniker setzt eine neue Batterie in das Gerät ein.
The technician inserts a new battery into the device.