The n-declination with some nouns occur with male singulars in dative, genitive and accusative, but only in certain groups.
It is called n-declination, but in fact the ending can also be -en.
The groups are (all male):
male nouns ending with -e male occupations and animals male nationalitie ending -ist ending -ent ending -ant ending – at, -et, -it, -ot
Examples:
Er untersucht den Patienten. He exams the patient. Er wohnt mit einem Polen zusammen. He lives with a polish man. Kennst du den neuen Praktikanten? Do you know the new trainee?