
zu
to
Expresses direction toward a place, a person, or a thing. It is used with the dative.
Ich gehe heute Abend zu meiner Freundin.
I'm going to my friend's place this evening.
to / too / closed
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to
Expresses direction toward a place, a person, or a thing. It is used with the dative.
Ich gehe heute Abend zu meiner Freundin.
I'm going to my friend's place this evening.

too
Used as an adverb to express an excessive amount or a high degree. It means 'more than is good or necessary.'
Die Suppe ist mir zu salzig.
The soup is too salty for me.

to + infinitive
Comes before a verb in the infinitive to express a purpose, intention, or necessity. It is often part of the construction 'um ... zu' (in order to).
Ich habe vergessen, die Blumen zu gießen.
I forgot to water the flowers.

closed
Used as an adverb and means 'closed.'
Mach bitte die Tür zu, es ist kalt.
Please close the door; it's cold.

for / at
Indicates a time or occasion, often with holidays or meals.
Was wünschst du dir zu Weihnachten?
What do you want for Christmas?

at home
A fixed expression that describes the place where someone lives.
Am Wochenende bleibe ich am liebsten zu Hause.
On the weekend, I prefer to stay at home.

by / on
Describes the way or manner in which something is done.
Wir gehen zu Fuß in die Stadt.
We're going into town on foot.

to
Indicates a ratio or comparison, often in sports scores or mixing ratios.
Die Heimmannschaft gewann das Spiel mit drei zu eins.
The home team won the game three to one.
When 'zu' (to) expresses direction toward a person or place, it always requires the dative. The question for this is 'Zu wem?' (to whom?) or 'Wohin?' (where to?). Make sure to decline the article after 'zu' correctly.
Wir fahren am Wochenende zu unseren Großeltern.
We're driving to our grandparents' this weekend.
In usage, the preposition 'zu' (to) often contracts with the following article. 'Zu dem' becomes 'zum,' and 'zu der' becomes 'zur.' This is very common in both spoken and written language.
Ich gehe heute zum Arzt.
I'm going to the doctor today.
As an adverb, 'zu' (too) comes directly before an adjective or another adverb to express an excessive degree. It signals that a quality exceeds the desired or normal amount.
Dieser Kaffee ist mir zu stark.
This coffee is too strong for me.
'Zu' comes before the infinitive of a verb in many subordinate-clause constructions and after certain verbs. In sentences with 'um ... zu' (in order to), it introduces the purpose and stands directly before the infinitive at the end of the sentence.
Es ist wichtig, jeden Tag Deutsch zu lernen.
It's important to learn German every day.
In the meaning 'geschlossen' (closed), 'zu' behaves like a separable prefix. In a main clause, it stands at the end. It is often combined with verbs like 'sein' (to be) or 'machen' (to make/do).
Warum ist das Fenster zu?
Why is the window closed?
'Zu' is used to name an occasion or a meal. Typical phrases are 'zu Weihnachten' (for Christmas), 'zum Geburtstag' (for one's birthday), or 'zum Frühstück' (for breakfast).
Was gibt es heute zum Abendessen?
What's for dinner today?
The expression 'zu Hause' (at home) always describes a static location, so it answers the question 'Wo?' (where?). It means that someone is at the place where they live.
Er ist heute nicht im Büro, er arbeitet von zu Hause aus.
He's not in the office today; he's working from home.
The means of movement 'zu Fuß' (on foot) is a fixed expression. No article is used here, unlike with 'mit dem Auto' (by car) or 'mit dem Fahrrad' (by bicycle).
Da die U-Bahn nicht fährt, müssen wir zu Fuß gehen.
Since the subway isn't running, we have to go on foot.
To indicate a ratio, a mixing result, or a score, 'zu' is used. It stands between the two numbers or values being compared.
Mischen Sie Wasser und Saft im Verhältnis zwei zu eins.
Mix water and juice in a ratio of two to one.
in
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 zur Bäckerei, wartet aber draußen.
Mia is going to the bakery but waiting outside.
Mia geht in die Bäckerei, um Brot zu kaufen.
Mia is going into the bakery to buy bread.