A small group of masculine nouns, most of which denote living beings, have the ending -n or -en throughout the plural and in all singular cases except the nominative.
der Junge
den Jungen
des Jungen
dem Jungen
die Jungen
die Jungen
der Jungen
den Jungen
Weak masculine nouns have no ending in the singular if they are used without a determiner (ambiguity with plural):
Die Situation war für Arzt und Patient kritisch.
8 masculine nouns are irregular: they have -ns in the genitive singular.