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 tour guide we asked lots of questions was patient. — der Reiseleiter · Fragen stellen + Dativ
The woman I showed my passport to was a police officer. — der Pass · die Polizistin
The tourists who don't like the hotel are complaining. — die Touristen · gefallen + Dativ · sich beschweren
The couple we explained the way to came from Canada. — das Paar · der Weg · erklären
The guests the waiter is bringing the food to are hungry. — die Gäste · der Kellner · das Essen
The driver we gave a tip to was nice. — der Fahrer · das Trinkgeld
'The flat I live in' → die Wohnung, in der ich wohne. The preposition moves to the front of the clause.
The hotel we're staying in is by the sea. — das Hotel · übernachten · am Meer
The island we're on holiday on is small. — die Insel · Urlaub machen auf
The people I'm travelling with are my colleagues. — die Leute · reisen mit
The train we're taking is late. — der Zug · fahren mit · Verspätung haben
The town she comes from is in Italy. — die Stadt · kommen aus
The countries I've been to were all beautiful. — die Länder
'whose' → dessen (masculine, neuter) or deren (feminine, plural).
The hotel whose rooms are so small is cheap. — das Hotel · die Zimmer
The city whose museums are famous is full of tourists. — die Stadt · die Museen
The flight whose price was so high was very comfortable. — der Flug · der Preis
The travellers whose suitcases are missing are waiting at the counter. — die Reisenden · die Koffer · der Schalter
The island whose beaches are white is very popular. — die Insel · die Strände
The guidebook whose tips were very good only costs ten euros. — der Reiseführer · die Tipps
All the cases mixed. Work out what the pronoun does in its own clause before you write it.
The tourist I'm showing the way to is from Japan. — die Touristin · der Weg · zeigen
The tourist whose suitcase got lost is from Japan. — die Touristin · der Koffer · verloren gehen
The suitcase I packed is heavy. — der Koffer · packen
The trip I dream of is to Japan. — die Reise · träumen von
The people we met were nice. — die Leute · begegnen + Dativ
The country whose language I'm learning is Germany. — das Land · die Sprache