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.
My brother, who I often help, lives in Berlin. — der Bruder · helfen + Dativ
My grandma, who I write to every week, is 90 years old. — die Oma · schreiben + Dativ
The child I gave the book to is happy. — das Kind · das Buch · schenken · sich freuen
My parents, to whom I owe a lot, are visiting us tomorrow. — die Eltern · verdanken + Dativ
The friends who liked the party want to come back. — die Freunde · die Party · gefallen + Dativ
My sister, who the car belongs to, is on holiday. — die Schwester · das Auto · gehören + Dativ
'The flat I live in' → die Wohnung, in der ich wohne. The preposition moves to the front of the clause.
The friend I talked to on the phone is coming on Saturday. — die Freundin · telefonieren mit
The cousin I'm staying with lives in Hamburg. — der Cousin · übernachten bei
The children my son plays with are very nice. — die Kinder · der Sohn · spielen mit
The house my grandparents live in is very old. — das Haus · die Großeltern
My friends, who I've learned a lot from, live abroad now. — die Freunde · lernen von · im Ausland
The wedding we're invited to is in June. — die Hochzeit · einladen zu
'whose' → dessen (masculine, neuter) or deren (feminine, plural).
My friend, whose sister lives in Vienna, is from Austria. — der Freund · die Schwester
The neighbour whose cat sleeps at our place is very friendly. — die Nachbarin · die Katze
The baby whose name is Lena sleeps a lot. — das Baby · der Name
The grandparents whose house is by the lake are inviting us. — die Großeltern · das Haus · am See · einladen
My aunt, whose children are already grown up, travels a lot. — die Tante · die Kinder · erwachsen
Grandpa, whose stories everyone loves, tells one every evening. — der Opa · die Geschichten · erzählen
All the cases mixed. Work out what the pronoun does in its own clause before you write it.
The woman whose husband is a doctor is called Anna. — die Frau · der Mann
The woman I helped is called Anna. — helfen + Dativ
The man I saw yesterday is my teacher. — der Mann · der Lehrer
The children I read to are five years old. — die Kinder · vorlesen + Dativ
Children whose parents work eat at school. — die Eltern
The friend whose brother I work with is coming today. — die Freundin · der Bruder · arbeiten mit