
hinein
into
Expresses movement from outside to inside a room, container, or area. The perspective is outside the destination.
Schau mal durch das Fenster ins Zimmer hinein.
Take a look through the window into the room.
in / into
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into
Expresses movement from outside to inside a room, container, or area. The perspective is outside the destination.
Schau mal durch das Fenster ins Zimmer hinein.
Take a look through the window into the room.

into
Describes extension or continuation into a period of time or situation.
Wir haben bis tief in die Nacht hinein gefeiert.
We celebrated until deep into the night.
'Hinein' (in/into) is a directional adverb made up of 'hin' and 'ein'. 'Hin' describes movement away from the speaker, and 'ein' means into something.
Er ging langsam in die dunkle Höhle hinein.
He slowly went into the dark cave.
'Hinein' (in/into) can be combined with many verbs to form a new separable verb, e.g. 'hineingehen' (to go in), 'hineinschauen' (to look in). In a main clause, the prefix goes at the end; in a subordinate clause, the verb stays together.
Bitte gehen Sie schon mal ins Wartezimmer hinein.
Please go on into the waiting room.
You can recognize the spatial meaning (m01) from verbs of movement and location information. You can recognize the temporal meaning (m02) from time expressions such as 'Nacht' (night), 'Tag' (day), or 'Zukunft' (future).
Er kletterte vorsichtig in den Brunnen hinein.
He climbed carefully into the well.
'Hinein' (in/into) is often used together with a prepositional phrase that describes the destination of the movement more precisely. The adverb 'hinein' then reinforces the direction.
Die Katze sprang mit einem Satz in den Karton hinein.
The cat jumped into the box in one leap.
pure / in
Both „rein“ and „hinein“ can describe movement inward. „Rein“ is the colloquial short form and leaves the perspective open. „Hinein“ more clearly marks movement from a position outside the destination to the inside.

Wirf den Brief von hier aus in den Kasten hinein.
Toss the letter into the box from here.
Wirf den Brief einfach rein in den Kasten.
Just toss the letter right in the box.