einsetzen
to insert
To put something into an opening or position intended for it and secure it.
Der Handwerker hat gestern die neuen Fenster eingesetzt.
The tradesman inserted the new windows yesterday.

to insert / to begin / to use
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to insert
To put something into an opening or position intended for it and secure it.
Der Handwerker hat gestern die neuen Fenster eingesetzt.
The tradesman inserted the new windows yesterday.

'Einsetzen' is a separable verb. In main clauses, the prefix 'ein' goes at the end; in subordinate clauses, the verb stays together, and in the infinitive with 'zu', 'zu' stands between 'ein' and 'setzen'.
Der Regen setzt am Nachmittag ein.
The rain begins in the afternoon.
The verb consists of the prefix 'ein' and the verb 'setzen' (to set/put). The basic idea is to 'put something in' or 'place something in'. This literal meaning also helps you understand the figurative meanings, where something is brought into a system or situation.
Der Arzt setzte dem Patienten ein künstliches Kniegelenk ein.
The doctor inserted an artificial knee joint into the patient.
In the meaning of 'beginnen' (to begin), 'einsetzen' is often used for natural events or impersonal processes. Typical subjects are 'der Regen' (the rain), 'die Dämmerung' (twilight), or 'die Wirkung' (the effect) of a medication.
Als die Dunkelheit einsetzte, wurde es still im Wald.
When darkness began, it became quiet in the forest.
When it is about using resources, 'einsetzen' (to use) is often combined with objects such as 'Mittel' (means/resources), 'Technik' (technology), 'Personal' (staff), or 'Kräfte' (forces/personnel).
Das Unternehmen setzt moderne Technik ein, um effizienter zu arbeiten.
The company uses modern technology to work more efficiently.
In the meaning of 'sich engagieren' (to get involved/commit oneself), the verb is always reflexive ('sich einsetzen') and is used with the preposition 'für' and the accusative.
Die Organisation setzt sich für die Rechte von Kindern ein.
The organization advocates for children's rights.
When 'einsetzen' is used with words such as 'Geld' (money), 'Zeit' (time), 'Kraft' (strength/energy), or even 'Leben' (life), it means to risk or invest something in order to achieve a goal.
Sie setzte ihre ganze Energie für das Projekt ein.
She put all her energy into the project.
to set / put
'setzen' and 'einsetzen' can both mean that you put something in a place. 'setzen' is more general: you put or place something somewhere. 'einsetzen' is more specific when something goes into a matching opening, holder, or intended position. The words can be confused because both describe moving something to a place. If it is only about the location, 'setzen' is often enough. If something belongs exactly inside something or is being secured in place, 'einsetzen' is more natural.

Nach dem Batteriewechsel setzt Ali die neue Batterie in die Fernbedienung ein.
After changing the battery, Ali inserts the new battery into the remote control.
Vor dem Termin setzt du die Stühle bitte in einen Kreis.
Before the appointment, please put the chairs in a circle.