Pip, 27 bytes

aR`[zfs]|one|[ent][iwh]`s._ 

Takes input as a command-line argument. Try it online!

First stab at this. Simple regex replacement, inserts a space before each match of [zfs]|one|[ent][iwh].

Pip, 27 bytes

aR`[zfs]|one|[ent][iwh]`s._ 

Takes input as a command-line argument. Try it online!

Simple regex replacement, inserts a space before each match of [zfs]|one|[ent][iwh].

Jumping on the bandwagon of stealing borrowing Uriel's regex gives 23 bytes (with -s flag):

a@`..[eox]|[tse]?....`