What is the possessive form of Jonas?

Answer 1

Jonas'

Answer 2

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the word; for example, "Put it on the Jonas' desk."

Add an apostrophe s ('s) after the existing s at the end of the word; for example, "Put it on the Jonas's desk."

I personally write the singular possessive form based on how I say the possessive phrase. If you are a student, use the form that is preferred by your teacher.