Rules for genitive 's

's - Apostrophe, genitive -s

English:	German:
Ronny's brother	Ronnys Bruder and  Ronny's Bruder (since spelling reform)

Singular:

Add 's:

Examples:
Mandy's brother John plays football.
My teacher's name is ...

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Plural:

Add the apostrophe ' to regular plural forms:

Examples:
The girls' room is very nice.
The Smiths' car is black.

Add 's to irregular plural forms:

Examples:
The children's books are over there.
Men's clothes are on the third floor.

If there are multiple nouns, add an 's only to the last noun:
Peter and John's mother is a teacher.

Source: http://www.englisch-hilfen.de/en/grammar/genitiv.htm