seinen Koffer

his suitcase

→ sein
phrase Uncommon #20,402
his suitcase Seinen is the masculine accusative form of sein (his), used here because Koffer is the object of the verb.
GE: Er packt seinen Koffer.
EN: He is packing his suitcase.

Ich musste Tom seinen Koffer tragen.

I had to carry Tom’s suitcase.

Bob, helfen Sie mir, seinen Koffer in das Wohnzimmer zu tragen!

Bob, help me carry his suitcase into the living room!

Tom fragte Mary, wohin er seinen Koffer stellen sollte.

Tom asked Mary where he should put his suitcase.

Tom ließ Mary seinen Koffer tragen.

Tom let Mary carry his suitcase.

Tom stellte seinen Koffer ab.

Tom put down his suitcase.

Er stellte seinen Koffer ab.

He put down his suitcase.

Bob, hilf mir, seinen Koffer in das Wohnzimmer zu tragen.

Bob, help me carry his suitcase into the living room.

Ich hab hier seinen Koffer.

I have his suitcase here.

— Bullet Train (2022)

Tom stellte seinen Koffer neben Marias.

Tom put his suitcase next to Maria's.

Tom stopfte alles in seinen Koffer.

Tom stuffed everything into his suitcase.

Er stellte seinen Koffer auf sein Bett.

He put his suitcase on his bed.