I find it difficult to know when soi versus soi-même is used.

Question

Sometimes when the english meaning is oneself, I reach for soi-même but the correct choice is sometimes soi.  Any hints please?

Answer

Bonjour Jennifer,
Here are a few links that may help differentiate the issue for you.  I am providing he links in lieu of copying and pasting the info:
https://forum.wordreference.com/threads/soi-m%C3%AAme-lui-m%C3%AAme-elle-m%C3%AAme.331578/
http://www.languefrancaise.net/forum/viewtopic.php?id=6127
http://dictionnaire.reverso.net/francais-definition/soi-m%C3%AAme
https://www.collinsdictionary.com/dictionary/french-english/soi-m%C3%AAme
https://www.collinsdictionary.com/dictionary/french-english/soi
J'espère que ma réponse vous aiderait.
Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisé par le monde français depuis l’époque d’Hugues Capet

Ron (a non-native speaker)

Answer

Take the English sentence "One has to do it by one-self". In French this translates to "On doit le faire soi-même." The emphatic stress pronouns put emphasis on who is responsible for the action. You could say:

On doit le faire. -- One has to do it.
or
On doit le faire soi-même. -- One has to do it oneself.

In this case you always add "-même". "On doit le faire soi" is simply wrong because "soi" by itself is a reflexive pronoun (like "me", "se", etc.). To make it a stress pronoun, add the "-même". Here is an example, where "soi" is used in a pure reflexive sense and not as a stress pronoun (hence sans "-même"): On doit prendre soin de soi. (One has to take care of oneself).

I hope that helps, -- Chris (not a native speaker).