der Neffe
nephew
The son of one's brother or sister.
Mein Neffe aus Hamburg kommt uns am Wochenende besuchen.
My nephew from Hamburg is coming to visit us this weekend.

nephew
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nephew
The son of one's brother or sister.
Mein Neffe aus Hamburg kommt uns am Wochenende besuchen.
My nephew from Hamburg is coming to visit us this weekend.

'Neffe' (nephew) is a weak masculine noun (N-declension). This means it takes the ending '-n' in all cases except the nominative singular. This also applies to all plural forms.
Ich schreibe meinem Neffen eine E-Mail.
I am writing my nephew an email.
The word 'Neffe' (nephew) is almost always used with a possessive pronoun (mein, dein, sein, ihr, etc.) to clearly express the family relationship.
Sein Neffe hat gerade die Schule abgeschlossen.
His nephew has just finished school.
To clarify the exact family relationship when it is not clear from context, you can use the structure 'der Neffe von + dative' (the nephew of + dative).
Das ist Peter, der Neffe von meiner Kollegin.
This is Peter, my colleague's nephew.
uncle
'der Onkel' and 'der Neffe' both belong to family relationships. However, they describe different directions within the family. An uncle is the brother of a parent or the husband of an aunt. A nephew is the son of one's own brother or sister. Learners may confuse the words because both often occur in the same relationship: for the boy, the man is the uncle; for the man, the boy is the nephew. So you need to pay attention to whose perspective you are speaking from.

Ich helfe meinem Neffen am Morgen bei den Hausaufgaben.
I help my nephew with his homework in the morning.
Morgen besuchen wir meinen Onkel zu Hause.
Tomorrow we are visiting my uncle at home.