
in
Indicates a place where something or someone is located (question: where?). Used with the dative.
Die Bücher stehen im Regal.
The books are on the shelf.
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Indicates a place where something or someone is located (question: where?). Used with the dative.
Die Bücher stehen im Regal.
The books are on the shelf.

into / to
Indicates a direction in which something or someone is moving (question: where to?). Used with the accusative.
Wir gehen heute Abend ins Kino.
We're going to the cinema tonight.

Indicates a point in time, a duration, or a period of time.
Das Projekt muss in zwei Wochen fertig sein.
The project has to be finished in two weeks.

Describes a state, a manner, or a material.
Das Auto ist in einem sehr guten Zustand.
The car is in very good condition.
'In' is a two-way preposition. This means the case depends on the question: for a location (question 'Wo?' (where?)), you use the dative. For a direction (question 'Wohin?' (where to?)), you use the accusative.
Ich bin im (in dem) Park. (Wo? → Dativ)
I am in the park. (Where? → dative)
In spoken and written German, 'in' and the article are often contracted. The most important short forms are 'im' (instead of 'in dem') and 'ins' (instead of 'in das').
Wir treffen uns im Café.
We're meeting in the café.
'In' is used very frequently for time expressions. It is used for points in time in the future ('in einer Stunde' (in an hour)), for months and seasons ('im Mai' (in May), 'im Winter' (in winter)), or for years ('im Jahr 1990' (in the year 1990)).
Der Bus kommt in zehn Minuten.
The bus is coming in ten minutes.
'In' is used in many fixed expressions to describe a state or manner. Common examples are 'in Ordnung' (all right/okay), 'in Eile sein' (to be in a hurry), 'in Gefahr sein' (to be in danger), or 'in bar zahlen' (to pay in cash).
Keine Sorge, alles ist in Ordnung.
Don't worry, everything is all right.
to / too / closed
In and zu can both express direction toward a destination. In emphasizes that someone enters a room or area. Zu indicates the destination without saying whether the person goes inside. So use in when someone enters, and zu when they merely reach the destination.

Mia geht in die Bäckerei, um Brot zu kaufen.
Mia is going into the bakery to buy bread.
Mia geht zur Bäckerei, wartet aber draußen.
Mia is going to the bakery but waiting outside.