Translate into German — the point is the relative pronoun.
Translate each sentence into German, using the German words given after the dash. Only the relative pronoun is checked — with its preposition, straight after the comma (…, mit denen …) — so keep that comma. The model is shown afterwards to compare the rest.
English often drops 'who' or leaves the preposition at the end: 'the friend I helped', 'the child I gave it to'. German always needs the pronoun, and the verb decides the case.
The customer I sent the offer to has called. — der Kunde · das Angebot · schicken
The boss, who I'm thanking for the help, is very fair. — die Chefin · danken + Dativ · die Hilfe
The colleagues I trust work on the second floor. — die Kollegen · vertrauen + Dativ · im zweiten Stock
The team the project belongs to is very small. — das Team · das Projekt · gehören + Dativ
The applicants we turned down get an e-mail. — die Bewerber · absagen + Dativ
The intern I explained everything to learns fast. — die Praktikantin · erklären
'The flat I live in' → die Wohnung, in der ich wohne. The preposition moves to the front of the clause.
The company I work for is international. — die Firma · arbeiten bei
The colleague I share the office with is very quiet. — der Kollege · das Büro · teilen
The customers we get a lot of praise from are from Japan. — die Kunden · das Lob
The department she works in is very big. — die Abteilung
The partners we work with are based in Vienna. — die Partner · zusammenarbeiten mit · sitzen
The boss I'm going to see tomorrow is strict. — der Chef · gehen zu
'whose' → dessen (masculine, neuter) or deren (feminine, plural).
The colleague whose office is next to mine talks loudly on the phone. — der Kollege · das Büro
The company whose products we sell is based in Munich. — die Firma · die Produkte
The project whose end we're celebrating was difficult. — das Projekt · das Ende
The employees whose contracts are ending are looking for new jobs. — die Mitarbeiter · die Verträge · die Stellen
The boss whose ideas are always good leads the team. — die Chefin · die Ideen · leiten
The customer whose order is missing is annoyed. — der Kunde · die Bestellung
All the cases mixed. Work out what the pronoun does in its own clause before you write it.
The colleague I'm giving the report to is new. — die Kollegin · der Bericht
The colleague whose report I'm reading is new. — die Kollegin · der Bericht
The colleague who always helps is on holiday. — der Kollege
The job I'm applying for is in Berlin. — die Stelle · sich bewerben für
The customers we're replying to have been waiting a long time. — die Kunden · antworten + Dativ
The boss whose company is growing is happy. — der Chef · die Firma · wachsen