Every one
Every one can be substituted for ‘each one’ or ‘every single one’ and is referring to individuals in a group. It can be used to refer to people, animals or objects.
- Every one of them went to the bar. (Every single one of them)
- Every one of those girls got drunk. (One refers to girl)
Everyone
Everyone refers to people (as a group) and can be substituted with ‘everybody’. As one word we are focusing on the group as a whole not as individuals.
- Everyone went to the bar. (The whole group)
- Everyone I know is coming to the fiesta this Saturday. (Several people are coming, but they are being treated as a single group)
Libertad final note:
Remembering which one to use, try this simple but effective tip:
If you can put the word ‘single’ between every and one, and it works fine, then you need to use the two words ‘every one’.