How do I make a name possessive in German?
In English I would say: Today is Tobias' birthday. Or Amy's dog is sad.
How would I do this in German? Thank you.
No sorry I should have explained that. An apostrophe is used in place of an "s" to indicate the possessive when the name ends with an "s" as in "Tobias".
The apostrophe is not used in German to show possession. Technically the "s" ending is for the genitive case, which shows possession. If you changed this to something like "Today is the teacher's birthday", it would be "Heute ist der Geburtstag des Lehrers". Another way to express possession is to use the preposition "von" (i.e., "of"), such as "Heute ist der Geburtstag von Tobias".
@Nachdenklich you used an apostrophe in your second example: "Heute ist Tobias' Geburtstag" Is it a typo?
Actually there is an accurate use of the possessive, at least in America -- although it is changing due to autocorrect. We were taught in school in the 1970's that when you had a plural noun or word ending in -s that you had to place the possessive apostrophe after the s to avoid an awkward pronunciation. So, The cat's toy ( one cat has one toy). Or The cats' toys (multiple cats share the same toy). The same hold true for names, This is Jenna's book or This is Andres' book. Again, this is changing with the advent of auto correct where the program's default is Noun's so we have now the equally acceptable Andres's book and the cats's toy. If you say these out loud as a native speaker from my generation it sounds awkward with the double s's and misleading as the two ss merge and the plural is lost.
Hope that helps