Why 'jemand' and 'niemand' change their endings
In English, someone and nobody never change. You say someone called, I saw someone and I gave it to someone, always with the same word. German works differently. The pronouns jemand (someone), niemand (nobody) and irgendjemand (anyone at all) take case endings, just like the article der turns into den or dem.
Because dictionaries list only the basic form, many learners assume jemand and niemand are fixed words like their English equivalents. So they say Ich kenne niemand or Ich helfe jemand. In speech this often slips through, but in writing it sounds unfinished. Once you know the two endings, the pattern is easy to spot and use.
The ending depends on the role the word plays in the sentence:
- Subject (nominative): jemand, niemand, irgendjemand
- Direct object (accusative), also after für, ohne, gegen: jemanden, niemanden, irgendjemanden
- Indirect object (dative), after verbs like helfen, glauben, danken and prepositions like mit, von, bei, zu: jemandem, niemandem, irgendjemandem
- Possession (genitive): jemandes, niemandes. This is rare. Most people say von jemandem instead.
Rule of thumb: the endings copy the masculine article. den ends in -en, so the accusative is jemanden. dem ends in -em, so the dative is jemandem. If you would ask wen?, use -en. If you would ask wem?, use -em.
jemand is the everyday word for someone or somebody. As the subject it stays as it is. As an object it takes -en or -em.
niemand means nobody or no one and follows exactly the same pattern. Note that German uses niemand where English often says not ... anyone.
irgendjemand stresses that it does not matter who: anyone at all. It is simply irgend- plus jemand, so it takes the same endings.
In everyday speech, many Germans drop the endings, especially in the accusative. You will hear this and it is not really wrong:
In writing, emails and exams, use the full forms jemanden and jemandem. They are always correct, while the bare form can look careless.
Do not transfer this pattern to jedermann (everyone). It works differently. It only adds -s in the genitive, as in Das ist nicht jedermanns Sache (That's not everyone's cup of tea), and otherwise stays unchanged. There is no jedermannen or jedermannem. It also sounds formal, so in daily life jeder or alle is more common.
The informal irgendwer (somebody) also changes, but it follows wer: irgendwen, irgendwem. Don't mix the two patterns into forms like irgendjemen.
Treat jemand, niemand and irgendjemand like the masculine article: no ending for the subject, -en for the direct object (like den), -em for the dative (like dem). Ask yourself wen? or wem?, then add the matching ending. In speech the endings are sometimes dropped, but in writing always use them.