genitive mode

[anandsing]

I have a doubt about the genitive mode .

In which cases can the "s" be left out . For example when a word ends with an "s" we can only use the apostrophe for example : actress - which is correct "actress's book or actress' book " In case of classical names like : Archimedes or Jesus , is it opcional to put the " s " after the apostrophe or not ?

I hope I was clear .

I look forward to an explanation.

Thank you

Anand

[Tdol]

With foreign names, we generally don't put the -s after the apostrophe, so we'd say Jesus' sermon, etc.
With 'actress' most would leave the-s off.