In Vim, I'd like to go back a word. The opposite of `w`

Use b to go back a word.

You may also want to check out W and B to advance/go back a WORD (which consists of a sequence of non-blank characters separated with white space, according to :h WORD).

It helps for me to think of it as:

b to go to beginning of current or previous word

w to go the beginning of next word

e to go to the end of current or next word

ge to go the end of the previous word

Try :h word-motions for more details and how to combine them with operations.

use "b" to move back - just tested in vi - works fine.