
einlegen
to insert
To put something into an opening intended for it or in a specific place. This often relates to technical devices or media.
Ich muss eine neue SIM-Karte in mein Handy einlegen.
I need to insert a new SIM card into my phone.
to insert / pickle
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to insert
To put something into an opening intended for it or in a specific place. This often relates to technical devices or media.
Ich muss eine neue SIM-Karte in mein Handy einlegen.
I need to insert a new SIM card into my phone.

to pickle
To preserve food in a liquid such as vinegar, oil, or brine.
Im Herbst legen wir immer Gurken und Paprika für den Winter ein.
In the fall, we always pickle cucumbers and peppers for the winter.

to file an appeal/objection
To take an official legal remedy against a court or administrative decision.
Gegen das Urteil können Sie innerhalb von zwei Wochen Berufung einlegen.
You can file an appeal against the judgment within two weeks.

to take a break
To interrupt an activity in order to rest.
Nach zwei Stunden Arbeit sollten wir eine kurze Pause einlegen.
After two hours of work, we should take a short break.

to engage a gear
To select a specific gear in a vehicle with a manual transmission.
Um am Berg anzufahren, musst du den ersten Gang einlegen.
To start on a hill, you have to engage first gear.
'Einlegen' is a separable verb. The prefix 'ein-' goes to the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Er legt eine DVD in den Player ein.
He inserts a DVD into the player.
The verb is made up of 'ein' (in/into) and 'legen' (to lay/put). So the basic meaning is to put something into something else. Many of the meanings, including the figurative ones, can be derived from this.
Bitte lege die Wäsche in den Schrank.
Please put the laundry in the closet.
In the literal meaning, 'einlegen' (to insert) is often used for media or technical parts. Typical objects are: a CD, a DVD, a battery, or a SIM card.
Könntest du bitte neue Batterien in die Fernbedienung einlegen?
Could you please insert new batteries into the remote control?
In the context of food, 'einlegen' (to pickle/preserve) means that you preserve it. Typical combinations are 'Gurken einlegen' (to pickle cucumbers), 'Hering einlegen' (to pickle herring), or to pickle/preserve something 'in Öl' (in oil) or 'in Essig' (in vinegar).
Meine Großmutter hat früher immer Hering in eine Sahnesoße eingelegt.
My grandmother used to always preserve herring in a cream sauce.
In a formal or legal context, 'einlegen' (to file/lodge) is part of fixed expressions. You can say 'Widerspruch einlegen' (to file an objection), 'Berufung einlegen' (to file an appeal), or 'ein Veto einlegen' (to lodge a veto).
Der Anwalt riet seinem Mandanten, gegen das Urteil Widerspruch einzulegen.
The lawyer advised his client to file an objection against the judgment.
The expression 'eine Pause einlegen' (to take a break) is very common and simply means 'eine Pause machen' (to take a break). It is often used in the context of work, studying, or long trips.
Wir fahren schon drei Stunden, lass uns mal eine Kaffeepause einlegen.
We've been driving for three hours already; let's take a coffee break.
In connection with cars with manual transmission, the fixed expression 'einen Gang einlegen' (to engage a gear) is used. You can also 'den Rückwärtsgang einlegen' (engage reverse gear).
Du musst erst den Rückwärtsgang einlegen, bevor du ausparken kannst.
You have to engage reverse gear before you can back out of the parking space.
to insert / to begin / to use
Both words can describe putting an object into a designated opening. „Einsetzen“ focuses more on precise positioning and possible attachment, while „einlegen“ specifically refers to placing devices, cards, and other media inside something. For a medium placed in a device, „einlegen“ is therefore often the usual choice.

Lena legte die SIM-Karte in das Lesegerät ein.
Lena inserted the SIM card into the card reader.
Der Techniker setzte die SIM-Karte genau in die Halterung ein.
The technician inserted the SIM card precisely into the holder.