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.

'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 a direction or destination. The important difference is: 'in' fits when you go into a room, building, or area. It often emphasizes the inside. 'zu' is used when you move toward a person, place, or point without especially emphasizing going inside. You go 'zum Arzt' (to the doctor), 'zur Freundin' (to a female friend / girlfriend), or 'zur Schule' (to school), but you go 'in die Küche' (into the kitchen) or 'ins Büro' (into the office) when the inside is important. There is a risk of confusion especially with places like school, work, or the doctor, because in German fixed usage often determines the choice.

Morgen gehe ich in die Bibliothek, um ein Buch abzuholen.
Tomorrow I'm going to the library to pick up a book.
Nach der Arbeit fahre ich zu meiner Schwester.
After work, I'm driving to my sister's place.