du
you
Second-person singular personal pronoun in the nominative. It refers to the person being addressed in an informal or familiar situation.
Du bist mein bester Freund.
You are my best friend.

you
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you
Second-person singular personal pronoun in the nominative. It refers to the person being addressed in an informal or familiar situation.
Du bist mein bester Freund.
You are my best friend.

The pronoun 'du' (you) is used for informal address. You use it with friends, family members, children, and often also with colleagues.
Hallo Mama, wie geht es dir?
Hi Mom, how are you?
When 'du' (you) is the subject in the sentence, the conjugated verb in the present tense almost always ends in '-st'. This is an important signal for recognizing the second-person singular.
Du sprichst sehr gut Deutsch.
You speak German very well.
'Du' (you) is the form for the subject, or nominative. When the person being addressed is the object, you use 'dich' (you, accusative) or 'dir' (you/to you, dative).
Ich sehe dich.
I see you.
you / her / their
'du' and 'ihr' are both informal forms of address. The important difference is number: with 'du', you address one single person. With 'ihr', you address several people whom you address informally. They can be confused because both are used in familiar situations. So always pay attention to whether you are speaking to one person or to a group.

Du kommst heute etwas früher zum Kurs.
You are coming to the course a little earlier today.
Ihr habt die Aufgabe zusammen sehr gut gemacht.
You did the assignment together very well.