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.

'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
'enthalten' and 'einschließen' can both express that something is part of a whole. 'enthalten' is often used when you are directly describing the contents: a drink contains sugar, a letter contains information. 'einschließen' emphasizes more strongly that something belongs with it or is included in a group. It often sounds somewhat more formal or precise when describing the limits of an offer, price, or plan. Learners may confuse the words because both can mean 'to be included'. What is important: 'einschließen' also has other meanings, for example locking someone in; in this relation, however, it is about 'including as a part'.

Der Preis schließt Frühstück und die Nutzung des Schwimmbads ein.
The price includes breakfast and use of the swimming pool.
Der Bericht enthält alle wichtigen Zahlen vom letzten Monat.
The report contains all the important figures from last month.