
einschließen
to lock in
To lock someone or something inside a room or container so that they or it can no longer get out.
Er hat sich aus Versehen im Keller eingeschlossen.
He accidentally locked himself in the basement.
to lock in / include / enclose
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to lock in
To lock someone or something inside a room or container so that they or it can no longer get out.
Er hat sich aus Versehen im Keller eingeschlossen.
He accidentally locked himself in the basement.

to include
To include or contain something or someone as part of a whole.
Der Preis schließt Frühstück und Abendessen ein.
The price includes breakfast and dinner.

to enclose
To completely surround or enclose something.
Die alte Stadtmauer schließt den historischen Kern komplett ein.
The old city wall completely encloses the historic center.
'Einschließen' (to lock in / include / enclose) is a separable verb. The prefix 'ein-' comes at the end in main clauses; in subordinate clauses the verb stays together, and in the infinitive with 'zu', 'zu' comes between 'ein' and 'schließen'.
Der Museumswärter schließt die Besucher nach der Öffnungszeit ein.
The museum guard locks the visitors in after opening hours.
You can recognize the meaning from the object: when a person is 'eingeschlossen' (locked in), it means 'einsperren' (to lock up). When a price or an offer 'etwas einschließt' (includes something), it means 'beinhalten' (to contain/include). When a wall or fence 'etwas einschließt' (encloses something), it means 'umgeben' (to surround).
Er schloss den Brief in dem Paket ein.
He enclosed the letter in the package.
In the meaning 'einsperren' (to lock up), the verb is very often used reflexively: 'sich einschließen' (to lock oneself in). This means that someone locks themselves, intentionally or unintentionally, in a room.
Ich habe mich aus Versehen im Badezimmer eingeschlossen.
I accidentally locked myself in the bathroom.
In the meaning 'beinhalten' (to include/contain), the word is often used in contracts, offers, or services. Typical phrases are 'die Mehrwertsteuer einschließen' (to include VAT), 'den Service einschließen' (to include the service), or 'alle Kosten einschließen' (to include all costs).
Schließt dieser Preis auch die Lieferung mit ein?
Does this price also include delivery?
to contain / abstain
Both words can express that something is part of a whole. „Enthalten“ primarily refers to the content that is already present, while „einschließen“ emphasizes the scope or the inclusion in a whole more strongly. Use „enthalten“ for a neutral description of the contents and „einschließen“ when the idea of belonging is explicitly implied.

Das Reisepaket schließt den Flug und zwei Übernachtungen ein.
The travel package includes the flight and two nights' accommodation.
Das Reisepaket enthält den Flug und zwei Übernachtungen.
The travel package contains the flight and two nights' accommodation.